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.

Fixed a small mistake.
x86jit: Flip cc in stl* to avoid reg loads.

Unfortunately, this zero thing is now concerning me...
x86jit: Don't lie about ZERO when it's not an imm.
x86jit: Allow EBX sign extension for 32-bit.
x86jit: Optimize some cases of ins/ext.

They happen but are minor.
Merge pull request #7075 from unknownbrackets/jit-minor

x86jit: A few more minor optimizations, zero bugfix