x64Emitter: refactor to support longer opcodes

Also add some new SSE4 opcodes.
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).
minor optimizations
JIT64: optimized the Paired Singles implementation

Use new instructions where possible and avoid redundant moves where possible.