[Bug tree-optimization/126957] New: The convert for mul_carry_low_sum/mul_carry_cross_sum (mul_carry_low?) is not optional for gimple

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126957

            Bug ID: 126957
           Summary: The convert for mul_carry_low_sum/mul_carry_cross_sum
                    (mul_carry_low?) is not optional for gimple
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: internal-improvement
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

These all do:
```
/* Carry from low-sum overflow: (cast?) (hilo > low_sum) << N.
   No explicit type/width guard needed: mul_low_sum delegates to
   mul_cross_sum + mul_hi + mul_lolo, which provide deep structural
   constraints, and @0 ties the shift amount to the inner constants.  */
(match (mul_carry_low_sum @op0 @op1 @mul_hilo0 @mul_hilo1 @mul_hilo2 @0 @1)
  (lshift
    (convert? (gt
      @mul_hilo0
      (mul_low_sum @op0 @op1 @mul_hilo1 @mul_hilo2 INTEGER_CST@0
       INTEGER_CST@1)))
    INTEGER_CST@0))
/* Carry from cross-sum overflow: (cast?) (hilo > cross_sum) << N.
   Explicit guard required because mul_cross_sum is just (plus:c @0 @1)
   with no inherent type or halfwidth constraint.  */
(match (mul_carry_cross_sum @mul_hilo0 @mul_hilo1 @mul_hilo2 @0)
  (lshift
    (convert? (gt
      @mul_hilo0
      (mul_cross_sum @mul_hilo1 @mul_hilo2)))
    INTEGER_CST@0)
  (with { tree op_type = TREE_TYPE (@mul_hilo0); }
  (if (INTEGRAL_TYPE_P (op_type)
       && TYPE_UNSIGNED (op_type)
       && TYPE_PRECISION (op_type) % 2 == 0
       && tree_fits_uhwi_p (@0)
       && tree_to_uhwi (@0) == TYPE_PRECISION (op_type) / 2))))
/* Carry from addition overflow: (cast?) (a > a + b).
   :c on gt also matches the LT form: (cast?) (a + b < a).  */
(match (mul_carry_low @0 @1)
  (convert?
    (gt:c @0 (plus:c @1 @0)))
  (with { tree op_type = TREE_TYPE (@0); }
  (if (INTEGRAL_TYPE_P (op_type) && TYPE_UNSIGNED (op_type)))))

```

But in gimple the convert is not optional from a comparison (boolean type; 1bit
type) to a different integer type. So the optional `?` should be removed.