[ARM] GlobalISel: Support G_ICMP for i32 and pointers

Add support throughout the pipeline:
- mark as legal for s32 and pointers
- map to GPRs
- lower to a sequence of instructions, which moves 0 or 1 into the
  result register based on the flags set by a CMPrr

We have copied from FastISel a helper function which maps CmpInst
predicates into ARMCC codes. Ideally, we should be able to move it
somewhere that both FastISel and GlobalISel can use.

llvm-svn: 305672