[GlobalISel] Add G_ROTL and G_ROTR to right_identity_zero

Similar to `DAGCombiner::visitRotate`.

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.

Differential Revision: https://reviews.llvm.org/D109264