ARM & AArch64: teach LowerVSETCC that output type size may differ from input.

While various DAG combines try to guarantee that a vector SETCC
operation will have the same output size as input, there's nothing
intrinsic to either creation or LegalizeTypes that actually guarantees
it, so the function needs to be ready to handle a mismatch.

Fortunately this is easy enough, just extend or truncate the naturally
compared result.

I couldn't reproduce the failure in other backends that I know have
SIMD, so it's probably only an issue for these two due to shared
heritage.

Should fix PR21645.

llvm-svn: 228518