[CodeGen] Ensure callers of CreateStackTemporary use sensible alignments

In two instances of CreateStackTemporary we are sometimes promoting
alignments beyond the stack alignment. I have introduced a new function
called getReducedAlign that will return the alignment for the broken
down parts of illegal vector types. For example, on NEON a <32 x i8>
type is made up of two <16 x i8> types - in this case the sensible
alignment is 16 bytes, not 32.

In the legalization code wherever we create stack temporaries I have
started using the reduced alignments instead for illegal vector types.

I added a test to

  CodeGen/AArch64/build-one-lane.ll

that tries to insert an element into an illegal fixed vector type
that involves creating a temporary stack object.

Differential Revision: https://reviews.llvm.org/D80370