[InstCombine] Support nusw in icmp of two geps with same base

Proof: https://alive2.llvm.org/ce/z/BYNQ7s

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -786,7 +786,7 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
       return transformToIndexedCompare(GEPLHS, RHS, Cond, DL, *this);
     }
 
-    bool GEPsInBounds = GEPLHS->isInBounds() && GEPRHS->isInBounds();
+    GEPNoWrapFlags NW = GEPLHS->getNoWrapFlags() & GEPRHS->getNoWrapFlags();
     if (GEPLHS->getNumOperands() == GEPRHS->getNumOperands() &&
         GEPLHS->getSourceElementType() == GEPRHS->getSourceElementType()) {
       // If the GEPs only differ by one index, compare it.
@@ -814,15 +814,15 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
         return replaceInstUsesWith(I, // No comparison is needed here.
           ConstantInt::get(I.getType(), ICmpInst::isTrueWhenEqual(Cond)));
 
-      else if (NumDifferences == 1 && GEPsInBounds) {
+      else if (NumDifferences == 1 && NW.hasNoUnsignedSignedWrap()) {
         Value *LHSV = GEPLHS->getOperand(DiffOperand);
         Value *RHSV = GEPRHS->getOperand(DiffOperand);
         // Make sure we do a signed comparison here.
         return new ICmpInst(ICmpInst::getSignedPredicate(Cond), LHSV, RHSV);
       }
     }
 
-    if (GEPsInBounds || CmpInst::isEquality(Cond)) {
+    if (NW.hasNoUnsignedSignedWrap() || CmpInst::isEquality(Cond)) {
       // ((gep Ptr, OFFSET1) cmp (gep Ptr, OFFSET2)  --->  (OFFSET1 cmp OFFSET2)
       Value *L = EmitGEPOffset(GEPLHS, /*RewriteGEP=*/true);
       Value *R = EmitGEPOffset(GEPRHS, /*RewriteGEP=*/true);

llvm/test/Transforms/InstCombine/icmp-gep.ll

@@ -295,6 +295,30 @@ define i1 @test60(ptr %foo, i64 %i, i64 %j) {
   ret i1 %cmp
 }
 
+define i1 @test60_nusw(ptr %foo, i64 %i, i64 %j) {
+; CHECK-LABEL: @test60_nusw(
+; CHECK-NEXT:    [[GEP1_IDX:%.*]] = shl nsw i64 [[I:%.*]], 2
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[GEP1_IDX]], [[J:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr nusw i32, ptr %foo, i64 %i
+  %gep2 = getelementptr nusw i8, ptr %foo, i64 %j
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}
+
+define i1 @test60_nusw_inbounds(ptr %foo, i64 %i, i64 %j) {
+; CHECK-LABEL: @test60_nusw_inbounds(
+; CHECK-NEXT:    [[GEP1_IDX:%.*]] = shl nsw i64 [[I:%.*]], 2
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[GEP1_IDX]], [[J:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep1 = getelementptr nusw i32, ptr %foo, i64 %i
+  %gep2 = getelementptr inbounds i8, ptr %foo, i64 %j
+  %cmp = icmp ult ptr %gep1, %gep2
+  ret i1 %cmp
+}
+
 define i1 @test_gep_ult_no_inbounds(ptr %foo, i64 %i, i64 %j) {
 ; CHECK-LABEL: @test_gep_ult_no_inbounds(
 ; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr i32, ptr [[FOO:%.*]], i64 [[I:%.*]]