Fix AArch64 ABI conformance issue in SIMD code.

In the AArch64 ABI, the high (unused) DWORD of a 32-bit argument's
register is undefined, so it was incorrect to use 64-bit instructions
to transfer a JDIMENSION argument in the 64-bit NEON SIMD functions.
The code worked thus far only because the existing compiler optimizers
weren't smart enough to do anything else with the register in question,
so the upper 32 bits happened to be all zeroes.

The latest builds of Clang/LLVM have a smarter optimizer, and under
certain circumstances, it will attempt to load-combine adjacent 32-bit
integers from one of the libjpeg structures into a single 64-bit integer
and pass that 64-bit integer as a 32-bit argument to one of the SIMD
functions (which is allowed by the ABI, since the upper 32 bits of the
32-bit argument's register are undefined.)  This caused the
libjpeg-turbo regression tests to crash.

This patch tries to use the Wn registers whenever possible.  Otherwise,
it uses a zero-extend instruction to avoid using the upper 32 bits of
the 64-bit registers, which are not guaranteed to be valid for 32-bit
arguments.

Based on sebpop@1fbae13

Closes #91.  Refer also to android-ndk/ndk#110 and
https://llvm.org/bugs/show_bug.cgi?id=28393

(cherrypick from external/libjpeg-turbo of c8282be605a139b269f2b8f4b4fb6f4118e740ab.)

Change-Id: I935a3696106e7ffcf9b1a6c12cb7f31d95b4ccc8

jsimd_arm64_neon.S

diff --git a/jsimd_arm64_neon.S b/jsimd_arm64_neon.S
index 099d4b5..26a8b11 100644
--- a/jsimd_arm64_neon.S
+++ b/jsimd_arm64_neon.S
@@ -237,6 +237,11 @@
     TMP3            .req x2
     TMP4            .req x15
 
+    /* OUTPUT_COL is a JDIMENSION (unsigned int) argument, so the ABI doesn't
+       guarantee that the upper (unused) 32 bits of x3 are valid.  This
+       instruction ensures that those bits are set to zero. */
+    uxtw x3, w3
+
     ROW0L           .req v16
     ROW0R           .req v17
     ROW1L           .req v18
@@ -794,6 +799,11 @@
     TMP4            .req x22
     TMP5            .req x23
 
+    /* OUTPUT_COL is a JDIMENSION (unsigned int) argument, so the ABI doesn't
+       guarantee that the upper (unused) 32 bits of x3 are valid.  This
+       instruction ensures that those bits are set to zero. */
+    uxtw x3, w3
+
     /* Load and dequantize coefficients into NEON registers
      * with the following allocation:
      *       0 1 2 3 | 4 5 6 7
@@ -1167,6 +1177,11 @@
     TMP3            .req x2
     TMP4            .req x15
 
+    /* OUTPUT_COL is a JDIMENSION (unsigned int) argument, so the ABI doesn't
+       guarantee that the upper (unused) 32 bits of x3 are valid.  This
+       instruction ensures that those bits are set to zero. */
+    uxtw x3, w3
+
     /* Save all used NEON registers */
     sub             sp, sp, 272
     str             x15, [sp], 16
@@ -1362,6 +1377,12 @@
     TMP1            .req x0
     TMP2            .req x15
 
+   /* OUTPUT_COL is a JDIMENSION (unsigned int) argument, so the ABI doesn't
+       guarantee that the upper (unused) 32 bits of x3 are valid.  This
+       instruction ensures that those bits are set to zero. */
+    uxtw x3, w3
+
+
     /* vpush           {v8.4h - v15.4h}            ; not available */
     sub             sp, sp, 208
     str             x15, [sp], 16
@@ -1709,11 +1730,11 @@
     .short          -128,  -128,   -128,   -128
 
 asm_function jsimd_ycc_\colorid\()_convert_neon
-    OUTPUT_WIDTH    .req x0
+    OUTPUT_WIDTH    .req w0
     INPUT_BUF       .req x1
-    INPUT_ROW       .req x2
+    INPUT_ROW       .req w2
     OUTPUT_BUF      .req x3
-    NUM_ROWS        .req x4
+    NUM_ROWS        .req w4
 
     INPUT_BUF0      .req x5
     INPUT_BUF1      .req x6
@@ -1723,7 +1744,7 @@
     Y               .req x8
     U               .req x9
     V               .req x10
-    N               .req x15
+    N               .req w15
 
     sub             sp, sp, 336
     str             x15, [sp], 16
@@ -1760,11 +1781,10 @@
     cmp             NUM_ROWS, #1
     b.lt            9f
 0:
-    lsl             x16, INPUT_ROW, #3
-    ldr             Y, [INPUT_BUF0, x16]
-    ldr             U, [INPUT_BUF1, x16]
+    ldr             Y, [INPUT_BUF0, INPUT_ROW, uxtw #3]
+    ldr             U, [INPUT_BUF1, INPUT_ROW, uxtw #3]
     mov             N, OUTPUT_WIDTH
-    ldr             V, [INPUT_BUF2, x16]
+    ldr             V, [INPUT_BUF2, INPUT_ROW, uxtw #3]
     add             INPUT_ROW, INPUT_ROW, #1
     ldr             RGB, [OUTPUT_BUF], #8