[SCEV] Fix incorrect extension in computeConstantDifference()

The Mul factor was zero-extended here, resulting in incorrect
results for integers larger than 64-bit.

As we currently only multiply by 1 or -1, just split this into
two cases -- there's no need for a full multiplication here.

Fixes llvm/llvm-project#102597.

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11976,9 +11976,14 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
   SmallDenseMap<const SCEV *, int, 8> Multiplicity;
   APInt Diff(BW, 0);
   auto Add = [&](const SCEV *S, int Mul) {
-    if (auto *C = dyn_cast<SCEVConstant>(S))
-      Diff += C->getAPInt() * Mul;
-    else
+    if (auto *C = dyn_cast<SCEVConstant>(S)) {
+      if (Mul == 1) {
+        Diff += C->getAPInt();
+      } else {
+        assert(Mul == -1);
+        Diff -= C->getAPInt();
+      }
+    } else
       Multiplicity[S] += Mul;
   };
   auto Decompose = [&](const SCEV *S, int Mul) {

llvm/test/Transforms/IndVarSimplify/pr102597.ll

@@ -1,14 +1,15 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
 ; RUN: opt -S -passes=indvars < %s | FileCheck %s
 
-; FIXME: This is a miscompile.
+; The %tobool condition should not be optimized away.
 define void @test() {
 ; CHECK-LABEL: define void @test() {
 ; CHECK-NEXT:  [[ENTRY:.*]]:
 ; CHECK-NEXT:    br label %[[LOOP:.*]]
 ; CHECK:       [[LOOP]]:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i128 [ 3, %[[ENTRY]] ], [ [[IV_DEC:%.*]], %[[LOOP_LATCH:.*]] ]
-; CHECK-NEXT:    br i1 true, label %[[LOOP_LATCH]], label %[[IF:.*]]
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i128 [[IV]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL]], label %[[LOOP_LATCH]], label %[[IF:.*]]
 ; CHECK:       [[IF]]:
 ; CHECK-NEXT:    call void @foo()
 ; CHECK-NEXT:    br label %[[LOOP_LATCH]]