[RISCV][GISel] Remove unnecessary Observer notifications from legalizeVAStart.

MIRBuilder already tells the observer when an instruction is created.

No other legalizer code on any target tells the observer when it
erases the instruction it was asked to legalize.

Author: topperc

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp

@@ -331,8 +331,7 @@ bool RISCVLegalizerInfo::legalizeShlAshrLshr(
 }
 
 bool RISCVLegalizerInfo::legalizeVAStart(MachineInstr &MI,
-                                         MachineIRBuilder &MIRBuilder,
-                                         GISelChangeObserver &Observer) const {
+                                         MachineIRBuilder &MIRBuilder) const {
   // Stores the address of the VarArgsFrameIndex slot into the memory location
   assert(MI.getOpcode() == TargetOpcode::G_VASTART);
   MachineFunction *MF = MI.getParent()->getParent();
@@ -341,10 +340,8 @@ bool RISCVLegalizerInfo::legalizeVAStart(MachineInstr &MI,
   LLT AddrTy = MIRBuilder.getMRI()->getType(MI.getOperand(0).getReg());
   auto FINAddr = MIRBuilder.buildFrameIndex(AddrTy, FI);
   assert(MI.hasOneMemOperand());
-  MachineInstr *LoweredMI = MIRBuilder.buildStore(
-      MI.getOperand(0).getReg(), FINAddr, *MI.memoperands()[0]);
-  Observer.createdInstr(*LoweredMI);
-  Observer.erasingInstr(MI);
+  MIRBuilder.buildStore(MI.getOperand(0).getReg(), FINAddr,
+                        *MI.memoperands()[0]);
   MI.eraseFromParent();
   return true;
 }
@@ -390,7 +387,7 @@ bool RISCVLegalizerInfo::legalizeCustom(LegalizerHelper &Helper,
     return true;
   }
   case TargetOpcode::G_VASTART:
-    return legalizeVAStart(MI, MIRBuilder, Observer);
+    return legalizeVAStart(MI, MIRBuilder);
   }
 
   llvm_unreachable("expected switch to return");

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.h

@@ -36,8 +36,7 @@ class RISCVLegalizerInfo : public LegalizerInfo {
   bool legalizeShlAshrLshr(MachineInstr &MI, MachineIRBuilder &MIRBuilder,
                            GISelChangeObserver &Observer) const;
 
-  bool legalizeVAStart(MachineInstr &MI, MachineIRBuilder &MIRBuilder,
-                       GISelChangeObserver &Observer) const;
+  bool legalizeVAStart(MachineInstr &MI, MachineIRBuilder &MIRBuilder) const;
 };
 } // end namespace llvm
 #endif