This patch implements the suggested codegen from PR24475:
but only for the fmaxf() case to start, so we can sort out any bugs before extending to fmin, f64, and vectors.
The fmax / maxnum definitions provide us flexibility for signed zeros, so I hope the only thing we have to worry about in this replacement sequence is NaN handling.
Note 1: I initially implemented this as lowerFMAXNUM(), but that exposes a problem: SelectionDAGBuilder::visitSelect() transforms compare/select instructions into FMAXNUM nodes if we declare FMAXNUM legal or custom. Perhaps this should be checking for NaN inputs or global unsafe-math before transforming? As it stands, this bypasses a big set of optimizations that the x86 backend already has in PerformSELECTCombine(). I don't know what the tradeoffs are for making a 'combine' rather than a 'lower'. If a 'lower' is preferred, we will need to fix that problem.
Note 2: The v2f32 test reveals another bug; the vector is extended to v4f32, so we have completely unnecessary operations happening on undef elements of the vector.