[NFC] [IndVarSimplify] Add non-overflowing usub test (llvm#159683)

We would reenter the loop with %i.04 being 0, so the usub would
overflow to -1. This was the only test in this file that had
an overflow in the loop, the other ones did not.

Author: fmayer

llvm/test/Transforms/IndVarSimplify/X86/overflow-intrinsics.ll

@@ -109,6 +109,37 @@ for.body:                                         ; preds = %entry, %cont
   %0 = tail call { i32, i1 } @llvm.usub.with.overflow.i32(i32 %i.04, i32 1)
   %1 = extractvalue { i32, i1 } %0, 1
 
+; CHECK: for.body:
+; CHECK-NOT: @llvm.usub.with.overflow.i32
+; CHECK: br i1 false, label %trap, label %cont, !nosanitize !0
+  br i1 %1, label %trap, label %cont, !nosanitize !{}
+
+trap:                                             ; preds = %for.body
+  tail call void @llvm.trap(), !nosanitize !{}
+  unreachable, !nosanitize !{}
+
+cont:                                             ; preds = %for.body
+  %2 = extractvalue { i32, i1 } %0, 0
+  %cmp = icmp sgt i32 %2, 0
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+}
+
+define void @f_usub_overflow(ptr nocapture %a) {
+; CHECK-LABEL: @f_usub_overflow(
+entry:
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %cont
+  ret void
+
+for.body:                                         ; preds = %entry, %cont
+  %i.04 = phi i32 [ 15, %entry ], [ %2, %cont ]
+  %idxprom = sext i32 %i.04 to i64
+  %arrayidx = getelementptr inbounds i8, ptr %a, i64 %idxprom
+  store i8 0, ptr %arrayidx, align 1
+  %0 = tail call { i32, i1 } @llvm.usub.with.overflow.i32(i32 %i.04, i32 1)
+  %1 = extractvalue { i32, i1 } %0, 1
+
 ; It is theoretically possible to prove this, but SCEV cannot
 ; represent non-unsigned-wrapping subtraction operations.