JIT: some paired singles optimizations
JIT: various float optimizations
JIT: Various JitAsmCommon optimizations

Use some SSE4 instructions in on CPUs that support them.
Use float instructions instead of int where appropriate (it's a cycle faster
on CPUs with arithmetic unit forwarding penalties).
JIT: add fselx implementation

Based on a patch by Tilka.
Merge pull request #686 from FioraAeterna/fiora

JIT: Optimize JitAsmCommon, Float, and PS implementations