[ARM] Fix invalid VLDM/VSTM access when targeting Big Endian with NEON

The instructions VLDM/VSTM can only access word-aligned memory
locations and produce alignment fault if the condition is not met.

The compiler currently generates VLDM/VSTM for v2f64 load/store
regardless the alignment of the memory access. Instead, if a v2f64
load/store is not word-aligned, the compiler should generate
VLD1/VST1. For each non double-word-aligned VLD1/VST1, a VREV
instruction should be generated when targeting Big Endian.

Differential Revision: https://reviews.llvm.org/D25281

llvm-svn: 283763