x86jit: Oops, fix a typesafety error.
x86jit: Use R15 when safe for the jit.

This is virtually always safe.
x86jit: Shave off a couple bytes in asm.
x86jit: Fix an x32 compile error in disabled code.
Merge pull request #7192 from unknownbrackets/jit-minor

x86jit: Free up a reg and cut some bytes from asm