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.

Author: petar-avramovic

llvm/lib/Target/AMDGPU/AMDGPURegBankLegalize.cpp

@@ -435,7 +435,8 @@ bool AMDGPURegBankLegalize::runOnMachineFunction(MachineFunction &MF) {
     unsigned Opc = MI->getOpcode();
     // Insert point for use operands needs some calculation.
     if (Opc == AMDGPU::G_PHI) {
-      RBLHelper.applyMappingPHI(*MI);
+      if (!RBLHelper.applyMappingPHI(*MI))
+        return false;
       continue;
     }
 
@@ -466,7 +467,8 @@ bool AMDGPURegBankLegalize::runOnMachineFunction(MachineFunction &MF) {
       // S1 rules are in RegBankLegalizeRules.
     }
 
-    RBLHelper.findRuleAndApplyMapping(*MI);
+    if (!RBLHelper.findRuleAndApplyMapping(*MI))
+      return false;
   }
 
   // Sgpr S1 clean up combines: