[ARM] Sink splat to ICmp

This adds ICmp to the list of instructions that we sink a splat to in a
loop, allowing the register forms of instructions to be selected more
often. It does not add FCmp yet as the results look a little odd, trying
to keep the register in an float reg and having to move it back to a GPR.

Differential Revision: https://reviews.llvm.org/D70997