[InstCombine] Add missing constant check (#170068)

`cast<Constant>` is not guarded by a type check during canonicalization
of predicates. This patch adds a type check in the outer if to avoid the
crash. `dyn_cast` may introduce another nested if, so I just use
`isa<Constant>` instead.

Address the crash reported in
#153053 (comment).

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1163,6 +1163,7 @@ static Value *canonicalizeSaturatedAddSigned(ICmpInst *Cmp, Value *TVal,
   // (X >= Y) ? INT_MAX : (X + C) --> sadd.sat(X, C)
   // where Y is INT_MAX - C or INT_MAX - C - 1, and C > 0
   if ((Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE) &&
+      isa<Constant>(Cmp1) &&
       match(FVal, m_Add(m_Specific(Cmp0), m_StrictlyPositive(C)))) {
     APInt IntMax =
         APInt::getSignedMaxValue(Cmp1->getType()->getScalarSizeInBits());

llvm/test/Transforms/InstCombine/saturating-add-sub.ll

@@ -2671,3 +2671,19 @@ define i8 @neg_neg_constant(i8 %x, i8 %y) {
   %s = select i1 %cmp, i8 127, i8 %d
   ret i8 %s
 }
+
+; Make sure we don't crash in this case.
+define i32 @pr153053_strict_pred_with_nonconstant_rhs(i32 %x, i32 %y) {
+; CHECK-LABEL: @pr153053_strict_pred_with_nonconstant_rhs(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[X]], 1
+; CHECK-NEXT:    [[RES:%.*]] = select i1 [[CMP]], i32 [[ADD]], i32 2147483647
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cmp = icmp slt i32 %x, %y
+  %add = add i32 %x, 1
+  %res = select i1 %cmp, i32 %add, i32 2147483647
+  ret i32 %res
+}