JIT: Handle GTF_REVERSE_OPS in fgMorphModToSubMulDiv (#71759)

We may see this in stress modes, and in general the JIT should be able to handle the flag.

src/coreclr/jit/morph.cpp

@@ -13741,10 +13741,12 @@ GenTree* Compiler::fgMorphModToSubMulDiv(GenTreeOp* tree)
 
  GenTreeOp* const div = tree;
 
- assert(!div->IsReverseOp());
-
- GenTree* dividend = div->gtGetOp1();
- GenTree* divisor = div->gtGetOp2();
+ GenTree* opA = div->gtGetOp1();
+ GenTree* opB = div->gtGetOp2();
+ if (div->IsReverseOp())
+ {
+ std::swap(opA, opB);
+ }
 
  TempInfo tempInfos[2];
  int tempInfoCount = 0;
@@ -13754,43 +13756,47 @@ GenTree* Compiler::fgMorphModToSubMulDiv(GenTreeOp* tree)
  // dividend are arbitrary nodes. For instance, if we spill the divisor and
  // the dividend is a local, we need to spill the dividend too unless the
  // divisor could not cause it to be reassigned.
+ // There is even more complexity due to needing to handle GTF_REVERSE_OPS.
+ //
  // This could be slightly better via GTF_CALL and GTF_ASG checks on the
  // divisor but the diffs of this were minor and the extra complexity seemed
  // not worth it.
- bool spillDividend;
- bool spillDivisor;
- if (divisor->IsInvariant() || divisor->OperIsLocal())
+ bool spillA;
+ bool spillB;
+ if (opB->IsInvariant() || opB->OperIsLocal())
  {
- spillDivisor  = false;
- spillDividend = !dividend->IsInvariant() && !dividend->OperIsLocal();
+ spillB = false;
+ spillA = !opA->IsInvariant() && !opA->OperIsLocal();
  }
  else
  {
- spillDivisor  = true;
- spillDividend = !dividend->IsInvariant();
+ spillB = true;
+ spillA = !opA->IsInvariant();
  }
 
- if (spillDividend)
+ if (spillA)
  {
- tempInfos[tempInfoCount] = fgMakeTemp(dividend);
- dividend = tempInfos[tempInfoCount].load;
+ tempInfos[tempInfoCount] = fgMakeTemp(opA);
+ opA  = tempInfos[tempInfoCount].load;
  tempInfoCount++;
  }
 
- if (spillDivisor)
+ if (spillB)
  {
- tempInfos[tempInfoCount] = fgMakeTemp(divisor);
- divisor = tempInfos[tempInfoCount].load;
+ tempInfos[tempInfoCount] = fgMakeTemp(opB);
+ opB  = tempInfos[tempInfoCount].load;
  tempInfoCount++;
  }
 
- var_types type = div->gtType;
+ GenTree* dividend = div->IsReverseOp() ? opB : opA;
+ GenTree* divisor = div->IsReverseOp() ? opA : opB;
 
  div->gtOp1 = gtClone(dividend);
  div->gtOp2 = gtClone(divisor);
 
- GenTree* const mul = gtNewOperNode(GT_MUL, type, div, divisor);
- GenTree* const sub = gtNewOperNode(GT_SUB, type, dividend, mul);
+ var_types type = div->gtType;
+ GenTree* const mul = gtNewOperNode(GT_MUL, type, div, divisor);
+ GenTree* const sub = gtNewOperNode(GT_SUB, type, dividend, mul);
 
  GenTree* result = sub;
  // We loop backwards as it is easier to create new commas