[avx] Define the _mm256_loadu2_xxx and _mm256_storeu2_xxx intrinsics.

From the Intel Optimization Reference Manual, Section 11.6.2.  When data cannot
be aligned or alignment is not known, 16-byte memory accesses may provide better
performance.
rdar://11076953


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@153091 91177308-0d34-0410-b5e6-96231b3b80d8
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