[TargetLowering] Teach computeRegisterProperties to only widen v3i16/v3f16...

[TargetLowering] Teach computeRegisterProperties to only widen v3i16/v3f16 vectors to the next power of 2 type if that's legal.

These were recently made simple types. This restores their
behavior back to something like their EVT legalization.

We might be able to fix the code in type legalization where the
assert was failing, but I didn't investigate too much as I had
already looked at the computeRegisterProperties code during the
review for v3i16/v3f16.

Most of the test changes restore the X86 codegen back to what
it looked like before the recent change. The test case in
vec_setcc.ll and is a reduced version of the reproducer from
the fuzzer.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=16490

llvm-svn: 369205