[ARM] Fix Neon vector type alignment to 64-bit

This is restricted version of patch - https://reviews.llvm.org/D33205
that I reverted as it was leading to ABI breaks on darwin etc.
This patch restricts the fix to AAPCS (Android remains 128-bit).
Reviewed by: Renato Golin, Stephen Hines
Differential Revision: https://reviews.llvm.org/D33786

llvm-svn: 304889