[LoopUnroll] Fix missing sign extension

For integers larger than 64-bit, this would zero-extend a -1
value, instead of sign-extending it.

Fixes llvm/llvm-project#80289.

Author: nikic

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -757,7 +757,7 @@ bool llvm::UnrollRuntimeLoopRemainder(
       !isGuaranteedNotToBeUndefOrPoison(TripCount, AC, PreHeaderBR, DT)) {
     TripCount = B.CreateFreeze(TripCount);
     BECount =
-        B.CreateAdd(TripCount, ConstantInt::get(TripCount->getType(), -1));
+        B.CreateAdd(TripCount, Constant::getAllOnesValue(TripCount->getType()));
   } else {
     // If we don't need to freeze, use SCEVExpander for BECount as well, to
     // allow slightly better value reuse.

llvm/test/Transforms/LoopUnroll/runtime-i128.ll

@@ -8,7 +8,7 @@ define void @test(i128 %n, i128 %m) {
 ; CHECK-SAME: i128 [[N:%.*]], i128 [[M:%.*]]) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = freeze i128 [[N]]
-; CHECK-NEXT:    [[TMP1:%.*]] = add i128 [[TMP0]], 18446744073709551615
+; CHECK-NEXT:    [[TMP1:%.*]] = add i128 [[TMP0]], -1
 ; CHECK-NEXT:    [[XTRAITER:%.*]] = and i128 [[TMP0]], 7
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i128 [[TMP1]], 7
 ; CHECK-NEXT:    br i1 [[TMP2]], label [[EXIT_UNR_LCSSA:%.*]], label [[ENTRY_NEW:%.*]]