[InstCombine] Do not modify GEP in place

This was modifying the GEP in place, with code to adjust the
inbounds flag. This was correct at the time, but now fails to
account for other GEP flags like nuw, leading to miscompilations.

Remove the special case, and always create a new GEP instruction.
Logic for preserving nuw in the cases where it is valid will be
added in a followup patch.

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2508,13 +2508,6 @@ Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
     if (Sum == nullptr)
       return nullptr;
 
-    // Update the GEP in place if possible.
-    if (Src->getNumOperands() == 2) {
-      GEP.setIsInBounds(isMergedGEPInBounds(*Src, *cast<GEPOperator>(&GEP)));
-      replaceOperand(GEP, 0, Src->getOperand(0));
-      replaceOperand(GEP, 1, Sum);
-      return &GEP;
-    }
     Indices.append(Src->op_begin()+1, Src->op_end()-1);
     Indices.push_back(Sum);
     Indices.append(GEP.op_begin()+2, GEP.op_end());

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -1905,5 +1905,17 @@ define ptr @gep_of_ashr_fail_not_divisible(ptr %p, i64 %x) {
   ret ptr %r
 }
 
+define ptr @gep_merge_not_nuw(ptr %p, i64 %idx) {
+; CHECK-LABEL: @gep_merge_not_nuw(
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[P:%.*]], i64 -1
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+  %idx.neg = sub i64 0, %idx
+  %add = add i64 %idx, -1
+  %gep1 = getelementptr inbounds i8, ptr %p, i64 %idx.neg
+  %gep = getelementptr inbounds nuw i8, ptr %gep1, i64 %add
+  ret ptr %gep
+}
+
 
 !0 = !{!"branch_weights", i32 2, i32 10}