[DAGCombiner][RISCV][AMDGPU] Call SimplifyDemandedBits at the end of...

[DAGCombiner][RISCV][AMDGPU] Call SimplifyDemandedBits at the end of visitMULHU to enable known bits contant folding.

We don't have real demanded bits support for MULHU, but we can
still use the known bits based constant folding support at the end
of SimplifyDemandedBits to simplify a MULHU. This helps with cases
where we know the LHS and RHS have enough leading zeros so that
the high multiply result is always 0.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D106471