[ARM GlobalISel] Make arm_i32imm an IntImmLeaf

This gets rid of some duplication in the TableGen definition, but it
forces us to keep both a pointer and a reference to the subtarget in the
ARMInstructionSelector. That is pretty ugly but it might be a reasonable
trade-off, since the TableGen descriptions should outlive the code in
the selector (or in the worst case we can update to use just the
reference when we get rid of DAGISel).

Differential Revision: https://reviews.llvm.org/D58031

llvm-svn: 355083