[ValueTracking] soften assert for invertible recurrence matching

There's a TODO comment in the code and discussion in D99912
about generalizing this, but I wasn't sure how to implement that,
so just going with a potential minimal fix to avoid crashing.

The test is a reduction beyond useful code (there's no user of
%user...), but it is based on https://llvm.org/PR50191, so this
is asserting on real code.

Differential Revision: https://reviews.llvm.org/D101772

Author: rotateright

llvm/lib/Analysis/ValueTracking.cpp

@@ -2608,7 +2608,8 @@ static Optional<unsigned> getInvertibleOperand(const Operator *Op1,
                                                   cast<Operator>(BO2));
     if (!Idx || *Idx != 0)
       break;
-    assert(BO1->getOperand(*Idx) == PN1 && BO2->getOperand(*Idx) == PN2);
+    if (BO1->getOperand(*Idx) != PN1 || BO2->getOperand(*Idx) != PN2)
+      break;
 
     // Phi operands might not be in the same order.  TODO: generalize
     // interface to return pair of operands.

llvm/test/Transforms/InstSimplify/icmp.ll

@@ -187,3 +187,29 @@ define i1 @shl_div_cmp_greater(i8 %x) {
   %cmp = icmp ule i8 %div, %x
   ret i1 %cmp
 }
+
+; Don't crash matching recurrences/invertible ops.
+
+define void @PR50191(i32 %x) {
+; CHECK-LABEL: @PR50191(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[P1:%.*]] = phi i32 [ [[X:%.*]], [[ENTRY:%.*]] ], [ [[SUB1:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[P2:%.*]] = phi i32 [ [[X]], [[ENTRY]] ], [ [[SUB2:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[SUB1]] = sub i32 [[P1]], [[P2]]
+; CHECK-NEXT:    [[SUB2]] = sub i32 42, [[P2]]
+; CHECK-NEXT:    br label [[LOOP]]
+;
+entry:
+  br label %loop
+
+loop:
+  %p1 = phi i32 [ %x, %entry ], [ %sub1, %loop ]
+  %p2 = phi i32 [ %x, %entry ], [ %sub2, %loop ]
+  %cmp = icmp eq i32 %p1, %p2
+  %user = zext i1 %cmp to i32
+  %sub1 = sub i32 %p1, %p2
+  %sub2 = sub i32 42, %p2
+  br label %loop
+}