InstSimplify: Start cleaning up simplifyFCmpInst

Also picks up a few improvements (Some of the fcmp.ll
test names imply they aren't quite testing what was intended.
Checking the sign bit can't be performed with a compare to a 0).

Much of the logic in here is the same as the class detection
logic of fcmpToClassTest. We could unify more with a weaker
version of fcmpToClassTest which returns implied classes rather
than exact class-like compares. Also could unify more with detection
of possible classes in non-splat vectors.

One problem here is we now only perform folds that used
to always work now require a context instruction. This is
because fcmpToClassTest requires the parent function.
Either fcmpToClassTest could tolerate a missing context
function, or we could require passing in one to simplifyFCmpInst.
Without this it's possible to hit the !isNan assert (which feels like
an unnecessary assert). In any case, these cases don't appear in
any tests.

https://reviews.llvm.org/D151887