Canonically identical types are allowed in compound expressions in C

We did not have a catch-all for when the two operand types are
identical after canonicalization. Instead, we handled that on a case by
case basis. Thus, we would diagnose code like:
```
mat4 test(int a) {
  typedef float mat4 __attribute((matrix_type(4, 4)));
  mat4 transform;
  return (a > 0) ? transform : transform;
}
```
This simplifies the logic and will be more forwards
compatible with other extended datatypes.

Fixes https://github.com/llvm/llvm-project/issues/69008