Fix alignment in AArch64InstructionSelector::emitConstantPoolEntry()

The code was using the alignment of a pointer to the value, not the
alignment of the constant itself.

Maybe we got away with it so far because the pointer alignment is
fairly high, but we did end up under-aligning <16 x i8> vectors,
which was caught in the Chromium build after lld stopped over-aligning
the .rodata.cst16 section in r356428. (See crbug.com/953815)

Differential revision: https://reviews.llvm.org/D61124

llvm-svn: 359287