Instead of JIT::Assembler making the decision for everyone and forcing
out every caller-saved register in the ABI onto the stack, we now leave
that decision to users of JIT::Assembler.
Instead of adjusting the stack pointer before/after making native calls,
just make sure we come out of enter() with the stack pointer aligned
for making calls.
This is strictly a code size reduction. :^)
This replaces the existing sized immediate operands with a unified
immediate operand that leaves the size handling to the assembler,
instead of the user.
This has 2 benefits:
1. The user doesn't need to know which specific operand size the
instruction expects when using it
2. The assembler automatically chooses the minimal operand size that
fits the given value, resulting in smaller code size without any
additional effort from the user. While the change is small, it still
has a noticeable effect on performance (since it increases the I$ hit
rate), resulting in 5% speedup on kraken a-star.
If a mov instruction is meant to be patchable, we don't want to rewrite
it as a xor, since that removes the slot where we'd patch in the right
value later.
Also, make sure to set both size bits in the REX prefix for xoring a
register with itself.
