[ARM GlobalISel] 64-bit memops should be aligned

We currently use only VLDR/VSTR for all 64-bit loads/stores, so the
memory operands must be word-aligned. Mark aligned operations as legal
and narrow non-aligned ones to 32 bits.

While we're here, also mark non-power-of-2 loads/stores as unsupported.

llvm-svn: 356872