[InstCombine] Use replaceOperand() API in GEP transforms

To make sure that replaced operands get DCEd. This drops one
iteration from gepphigep.ll, which is still not optimal.

This was the last test case performing more than 3 iterations.

NFC-ish, only worklist order should change.

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1996,7 +1996,7 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
 
     GEP.getParent()->getInstList().insert(
         GEP.getParent()->getFirstInsertionPt(), NewGEP);
-    GEP.setOperand(0, NewGEP);
+    replaceOperand(GEP, 0, NewGEP);
     PtrOp = NewGEP;
   }
 
@@ -2096,8 +2096,8 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
       // Update the GEP in place if possible.
       if (Src->getNumOperands() == 2) {
         GEP.setIsInBounds(isMergedGEPInBounds(*Src, *cast<GEPOperator>(&GEP)));
-        GEP.setOperand(0, Src->getOperand(0));
-        GEP.setOperand(1, Sum);
+        replaceOperand(GEP, 0, Src->getOperand(0));
+        replaceOperand(GEP, 1, Sum);
         return &GEP;
       }
       Indices.append(Src->op_begin()+1, Src->op_end()-1);
@@ -2215,9 +2215,8 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
             // array.  Because the array type is never stepped over (there
             // is a leading zero) we can fold the cast into this GEP.
             if (StrippedPtrTy->getAddressSpace() == GEP.getAddressSpace()) {
-              GEP.setOperand(0, StrippedPtr);
               GEP.setSourceElementType(XATy);
-              return &GEP;
+              return replaceOperand(GEP, 0, StrippedPtr);
             }
             // Cannot replace the base pointer directly because StrippedPtr's
             // address space is different. Instead, create a new GEP followed by

llvm/test/Transforms/InstCombine/gepphigep.ll

@@ -1,4 +1,4 @@
-; RUN: opt -instcombine -S  < %s | FileCheck %s
+; RUN: opt -instcombine -instcombine-infinite-loop-threshold=3 -S < %s | FileCheck %s
 
 %struct1 = type { %struct2*, i32, i32, i32 }
 %struct2 = type { i32, i32 }