[SVE] Fix a bug with vectorization external set.

This patch fixes a bug when LoopOptimization::vector_external_set_
(contains vector instructions to be inserted outside of newly
generated loops) was reset more than once during vectorization. Instead,
it should only be reset when vectorization of the loop has finished.

It also adds a regression test for the situation: a loop is vectorized
in predicated mode and a disambiguation test and a back-up scalar loop
are needed and inserted.

Note: This bug only affected predicated vectorization (e.g. using Arm
SVE) and didn't impact any production devices.

Test: run with ART_FORCE_TRY_PREDICATED_SIMD=true and without
Test: 623-checker-loop-regressions
Test: ./art/test.py --host --optimizing --jit
Test: ./art/test.py --target --optimizing --jit
Change-Id: I19fabd4d7034e5aa6421df79b2519d8f7e6ef43a