[flang] Don't create impossible conversions in intrinsic extension (#79042)

We support specific intrinsic calls like `AMAX0(1.0,2)` that have
heterogeneous argument types as an optional extension in cases where the
specific intrinsic has a related generic intrinsic function capable of
handling the argument types. This feature can't be allowed to apply to
calls where the result of the related generic intrinsic function is not
convertible to the type of the specific intrinsic, as in `AMAX0('a',
'b')`.

Fixes https://github.com/llvm/llvm-project/issues/78932.