AMDGPU/GlobalISel: Report RegBankLegalize errors using reportGISelFailure (#169918)
Use standard GlobalISel error reporting with reportGISelFailure
and pass returning false instead of llvm_unreachable.
Also enables -global-isel-abort=0 or 2 for -global-isel -new-reg-bank-select.
Note: new-reg-bank-select with abort 0 or 2 runs LCSSA,
while "intended use" without abort or with abort 1 does not run LCSSA.