[InstSimplify] Support non-inbounds GEP in ptrdiff fold (llvm#162676)

We can fold ptrdiff(ptradd(p, x), p) to x regardless of whether the
ptradd is inbounds.

Proof: https://alive2.llvm.org/ce/z/Xuvc7N

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -671,12 +671,12 @@ Value *llvm::simplifyAddInst(Value *Op0, Value *Op1, bool IsNSW, bool IsNUW,
 /// This is very similar to stripAndAccumulateConstantOffsets(), except it
 /// normalizes the offset bitwidth to the stripped pointer type, not the
 /// original pointer type.
-static APInt stripAndComputeConstantOffsets(const DataLayout &DL, Value *&V,
-                                            bool AllowNonInbounds = false) {
+static APInt stripAndComputeConstantOffsets(const DataLayout &DL, Value *&V) {
   assert(V->getType()->isPtrOrPtrVectorTy());
 
   APInt Offset = APInt::getZero(DL.getIndexTypeSizeInBits(V->getType()));
-  V = V->stripAndAccumulateConstantOffsets(DL, Offset, AllowNonInbounds);
+  V = V->stripAndAccumulateConstantOffsets(DL, Offset,
+                                           /*AllowNonInbounds=*/true);
   // As that strip may trace through `addrspacecast`, need to sext or trunc
   // the offset calculated.
   return Offset.sextOrTrunc(DL.getIndexTypeSizeInBits(V->getType()));

llvm/test/Transforms/InstSimplify/ptr_diff.ll

@@ -14,11 +14,7 @@ define i64 @ptrdiff(ptr %ptr) {
 
 define i64 @ptrdiff_no_inbounds(ptr %ptr) {
 ; CHECK-LABEL: @ptrdiff_no_inbounds(
-; CHECK-NEXT:    [[LAST:%.*]] = getelementptr i8, ptr [[PTR:%.*]], i32 42
-; CHECK-NEXT:    [[FIRST_INT:%.*]] = ptrtoint ptr [[PTR]] to i64
-; CHECK-NEXT:    [[LAST_INT:%.*]] = ptrtoint ptr [[LAST]] to i64
-; CHECK-NEXT:    [[DIFF:%.*]] = sub i64 [[LAST_INT]], [[FIRST_INT]]
-; CHECK-NEXT:    ret i64 [[DIFF]]
+; CHECK-NEXT:    ret i64 42
 ;
   %last = getelementptr i8, ptr %ptr, i32 42
   %first.int = ptrtoint ptr %ptr to i64