[AMDGPU] Experiments show that the GCNRegBankReassign pass significantly impacts

the compilation time and there is no case for which we see any improvement in
performance. This patch removes this pass and its associated test cases from
the tree.

Differential Revision: https://reviews.llvm.org/D101313

Change-Id: I0599169a7609c19a887f8d847a71e664030cc141