x86jit: Improve cvt.w.s when fd is loaded or fs.

We have no need to store it.
x86jit: Optimize neg.s and abs.s a tiny bit.

Same reg is probably a common case, improves micro benchmark.
x86jit: Very small optimization to c.nge.s.
x86jit: Small optimization for add.s f1, f2, f2.

Doubles the speed of that particular case.  Biggest difference is not
loading fd for no reason.
x86jit: Implement round.w.s and friends.

They are not terribly fast, though, updating MXCSR.
x86jit: Avoid speculative loads in mtc1/mfc1.
x86jit: Optimize loads of 0 into fp regs.
x86jit: Micro optimize slt* a bit.

This improves their performance and hopefully latency.  It also avoids
filling registers that are not likely to be used again.
Clarify a comment.
Merge pull request #7074 from unknownbrackets/jit-minor

x86jit: More fpu optimizations