ARM: check alignment before transforming ldr -> ldm (or similar).

ARM: check alignment before transforming ldr -> ldm (or similar).

ldm and stm instructions always require 4-byte alignment on the pointer, but we
weren't checking this before trying to reduce code-size by replacing a
post-indexed load/store with them. Unfortunately, we were also dropping this
incormation in DAG ISel too, but that's easy enough to fix.

llvm-svn: 281893