JIT64: use LEA for the "a = b + imm" case of addi
JIT64: tweak srwx/slwx

Register B gets immediately moved into te shift register, so even if a == b
it doesn't need to be loaded.
JIT64: Optimize carry handling

Carries are rather common and unpredictable, so do them branchlessly wherever
we can. Optimize some special cases of carries where possible.
JIT64: support merged branching for rlwinmx, too

Not quite as common a branch instruction as cmpwi, but close.