[InstCombine] replace negated operand in fcmp with 0.0

X (any pred) -X --> X (any pred) 0.0

This works with all FP values and preserves FMF.
Alive2 examples:
https://alive2.llvm.org/ce/z/dj6jhp

This can also create one of the patterns that we match as "fabs"
as shown in one of the test diffs.