[InstCombine] Increase recursion limit to 3 in simplifyNonNullOperand (#128695)

Address review comment
#128466 (comment)

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=72781f58efddecee19feb07fec4e6104ef4c4812&to=3853aee61626b0eda06671b4cbbc4cdd1344440c&stat=instructions:u

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -996,7 +996,8 @@ Value *InstCombinerImpl::simplifyNonNullOperand(Value *V,
   if (!V->hasOneUse())
     return nullptr;
 
-  if (Depth == 1)
+  constexpr unsigned RecursionLimit = 3;
+  if (Depth == RecursionLimit)
     return nullptr;
 
   if (auto *GEP = dyn_cast<GetElementPtrInst>(V)) {

llvm/test/Transforms/InstCombine/load.ll

@@ -451,3 +451,33 @@ define i32 @load_select_with_null_gep(i1 %cond, ptr %p, i64 %off) {
   %res = load i32, ptr %gep, align 4
   ret i32 %res
 }
+
+define i16 @load_select_with_null_gep2(i1 %cond, ptr %p, i64 %x) {
+; CHECK-LABEL: @load_select_with_null_gep2(
+; CHECK-NEXT:    [[INVARIANT_GEP:%.*]] = getelementptr i8, ptr [[SEL:%.*]], i64 -2
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i16, ptr [[INVARIANT_GEP]], i64 [[X:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = load i16, ptr [[GEP]], align 2
+; CHECK-NEXT:    ret i16 [[RES]]
+;
+  %sel = select i1 %cond, ptr %p, ptr null
+  %invariant.gep = getelementptr i8, ptr %sel, i64 -2
+  %gep = getelementptr i16, ptr %invariant.gep, i64 %x
+  %res = load i16, ptr %gep, align 2
+  ret i16 %res
+}
+
+define i16 @load_select_with_null_gep3(i1 %cond, ptr %p, i64 %x, i64 %y) {
+; CHECK-LABEL: @load_select_with_null_gep3(
+; CHECK-NEXT:    [[INVARIANT_GEP:%.*]] = getelementptr i8, ptr [[SEL:%.*]], i64 -2
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i16, ptr [[INVARIANT_GEP]], i64 [[X:%.*]]
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i16, ptr [[GEP]], i64 [[Y:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = load i16, ptr [[GEP2]], align 2
+; CHECK-NEXT:    ret i16 [[RES]]
+;
+  %sel = select i1 %cond, ptr %p, ptr null
+  %invariant.gep = getelementptr i8, ptr %sel, i64 -2
+  %gep = getelementptr i16, ptr %invariant.gep, i64 %x
+  %gep2 = getelementptr i16, ptr %gep, i64 %y
+  %res = load i16, ptr %gep2, align 2
+  ret i16 %res
+}