[AVR] Fix codegen after getConstant assertions got enabled (#152269)

This fixes #152097

This commit fixes two instances of a (somewhat) recently enabled
assertion. One with a test, the other I can't reproduce (might be dead
code) but certainly looks like an instance of the same problem.

The PR that introduced the regression:
#117558

With this patch, the AVR backend is usable again for TinyGo.

Author: aykevl

llvm/lib/Target/AVR/AVRISelLowering.cpp

@@ -669,7 +669,7 @@ SDValue AVRTargetLowering::getAVRCmp(SDValue LHS, SDValue RHS, ISD::CondCode CC,
       default: {
         // Turn lhs < rhs with lhs constant into rhs >= lhs+1, this allows
         // us to  fold the constant into the cmp instruction.
-        RHS = DAG.getConstant(C->getSExtValue() + 1, DL, VT);
+        RHS = DAG.getSignedConstant(C->getSExtValue() + 1, DL, VT);
         CC = ISD::SETGE;
         break;
       }
@@ -713,7 +713,10 @@ SDValue AVRTargetLowering::getAVRCmp(SDValue LHS, SDValue RHS, ISD::CondCode CC,
     // Turn lhs < rhs with lhs constant into rhs >= lhs+1, this allows us to
     // fold the constant into the cmp instruction.
     if (const ConstantSDNode *C = dyn_cast<ConstantSDNode>(RHS)) {
-      RHS = DAG.getConstant(C->getSExtValue() + 1, DL, VT);
+      // Doing a "icmp ugt i16 65535, %0" comparison should have been converted
+      // already to something else. Assert to make sure this assumption holds.
+      assert((!C->isAllOnes()) && "integer overflow in comparison transform");
+      RHS = DAG.getConstant(C->getZExtValue() + 1, DL, VT);
       CC = ISD::SETUGE;
       break;
     }

llvm/test/CodeGen/AVR/cmp.ll

@@ -298,3 +298,18 @@ define i16 @cmp_i16_gt_1023(i16 %0) {
   %3 = zext i1 %2 to i16
   ret i16 %3
 }
+
+define void @cmp_issue152097(i16 %a) addrspace(1) {
+; See: https://github.com/llvm/llvm-project/issues/152097
+; CHECK-LABEL: cmp_issue152097
+; CHECK:      ldi r18, -1
+; CHECK-NEXT: cpi r24, -2
+; CHECK-NEXT: cpc r25, r18
+; CHECK-NEXT: ret
+  %cmp = icmp ugt i16 -2, %a
+  br i1 %cmp, label %if.then, label %if.else
+if.then:
+  ret void
+if.else:
+  ret void
+}