[CodeGen] Prevent nullptr deref in genAlternativeCodeSequence

[fdeazeve]

Summary

A pointer dereference was added (D141302) above an assert that checks
whether the pointer is null. This commit moves the assert above the
dereference and transforms it into an llvm_unreachable to better express
the intent that certain switch cases should never be reached.

[fdeazeve]

Update commit message

[fhahn]

LGTM, thanks!

llvm/lib/CodeGen/TargetInstrInfo.cpp

void TargetInstrInfo::genAlternativeCodeSequence(
   case MachineCombinerPattern::REASSOC_XA_BY:
     Prev = MRI.getUniqueVRegDef(Root.getOperand(1).getReg());
     break;
   case MachineCombinerPattern::REASSOC_AX_YB:
   case MachineCombinerPattern::REASSOC_XA_YB:
     Prev = MRI.getUniqueVRegDef(Root.getOperand(2).getReg());
     break;
   default:
-    break;
+    llvm_unreachable("Unknown pattern for machine combiner");
   }
 
   // Don't reassociate if Prev and Root are in different blocks.
   if (Prev->getParent() != Root.getParent())
     return;
 
-  assert(Prev && "Unknown pattern for machine combiner");
-
   reassociateOps(Root, *Prev, Pattern, InsInstrs, DelInstrs, InstIdxForVirtReg);
 }
 
 MachineTraceStrategy TargetInstrInfo::getMachineCombinerTraceStrategy() const {
   return MachineTraceStrategy::TS_MinInstrCount;
 }
 
 bool TargetInstrInfo::isReallyTriviallyReMaterializableGeneric(