[DAGCombiner][AMDGPU] Canonicalize constants to the RHS of MULHU/MULHS.

This allows special constants like to 0 to be recognized. It's also
expected by isel patterns if a target had a mulh with immediate instructions.
The commuting done by tablegen won't commute patterns with immediates since it
expects DAGCombine to have done it.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D107486