[AMDGPU][RegBankCombiner] Add cast_of_cast and constant_fold_cast combines

We can add a bunch of exts/truncs during RBSelect, we should be able to fold
them away afterwards.

Author: Pierre-vh

llvm/lib/Target/AMDGPU/AMDGPUCombine.td

@@ -180,5 +180,6 @@ def AMDGPURegBankCombiner : GICombiner<
   [unmerge_merge, unmerge_cst, unmerge_undef,
    zext_trunc_fold, int_minmax_to_med3, ptr_add_immed_chain,
    fp_minmax_to_clamp, fp_minmax_to_med3, fmed3_intrinsic_to_clamp,
-   identity_combines, redundant_and]> {
+   identity_combines, redundant_and, constant_fold_cast_op,
+   cast_of_cast_combines]> {
 }