[AMDGPU] Drop the GCNRegBankReassign pass

if we can resolve reg bank conflict, some stall cycle may not happen.
so why the newest llvm version delete this pass?

commit link:

The commit message says “Experiments show that the GCNRegBankReassign pass significantly impacts the compilation time and there is no case for which we see any improvement in performance.”

If you can show that the pass ever provided a measurable performance improvement, then perhaps we can think about reinstating it.