[AMDGPU] Avoid redundant mode register writes

Summary:
The SIModeRegister pass attempts to generate the minimal number of
writes to the mode register. However it was failing to correctly
deal with some loops, resulting in some redundant setreg instructions
being inserted.

This change amends the pass to avoid generating these redundant
instructions.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82215