[X86][AVX] Ensure vector non-temporal load/store intrinsics force pointer alignment (PR33830)

Clang specifies a max type alignment of 16 bytes on darwin targets (annoyingly in the driver not via cc1), meaning that the builtin nontemporal stores don't correctly align the loads/stores to 32 or 64 bytes when required, resulting in lowering to temporal unaligned loads/stores.

This patch casts the vectors to explicitly aligned types prior to the load/store to ensure that the require alignment is respected.

Differential Revision: https://reviews.llvm.org/D35996

llvm-svn: 309488