Use vld1 / vst2 for unaligned v2f64 load / store. e.g. Use vld1.16 for 2-byte

aligned address. Based on patch by David Peixotto.

Also use vld1.64 / vst1.64 with 128-bit alignment to take advantage of alignment
hints. rdar://12090772, rdar://12238782

llvm-svn: 164089