armjit: Preserve imms on min/max as well.
armjit: optimize reverse subtract, avoid temp imms.

If we have a non-op2 imm, get rid of it asap.  If we have a op2 friendly
imm, keep it.
armjit: Improve GPR typesafety.
armjit: Allocate regs in preferred slots.

This may allow better flushing.  Not sure if these are the best regs,
but if they aren't it shouldn't really hurt.
armjit: Flush regs using STMIA where possible.
armjit: Clean up some magic numbers.
Merge pull request #4480 from unknownbrackets/perf

Flush regs using STMIA if possible, plus imm adjustments (armjit)