[InstCombine] convert FP min/max with negated op to fabs

This is part of improving floating-point patterns seen in:
https://llvm.org/PR39480

We don't require any FMF because the 2 potential corner cases
(-0.0 and NaN) are correctly handled without FMF:
1. -0.0 is treated as strictly less than +0.0 with
   maximum/minimum, so fabs/fneg work as expected.
2. +/- 0.0 with maxnum/minnum is indeterminate, so
   transforming to fabs/fneg is more defined.
3. The sign of a NaN may be altered by this transform,
   but that is allowed in the default FP environment.

If there are FMF, they are propagated from the min/max call to
one or both new operands which seems to agree with Alive2:
https://alive2.llvm.org/ce/z/bem_xC