[Sema] add warning for tautological FP compare with literal

If we are equality comparing an FP literal with a value cast from a type
where the literal can't be represented, that's known true or false and
probably a programmer error.

Fixes issue #54222.
https://github.com/llvm/llvm-project/issues/54222

Note - I added the optimizer change with:
9397bdc6
...and as discussed in the post-commit comments, that transform might be
too dangerous without this warning in place, so it was reverted to allow
this change first.

Differential Revision: https://reviews.llvm.org/D121306