[GlobalISel] Add G_ROTL and G_ROTR to right_identity_zero

[paquette]

Summary

Similar to DAGCombiner::visitRotate.

rotr x, 0 -> x
rotl x, 0 -> x

This makes rotl_bitwidth_cst in postlegalizercombiner-rotate.mir reduce down to a COPY. Modify the checkline to make sure that only rotate_out_of_range runs there.

[jroelofs]

LGTM

llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir

345

extra ...