JIT64: support merged branching for rlwinmx, too

Not quite as common a branch instruction as cmpwi, but close.
JIT64: optimize carry calculations

Omit carry calculations that get overwritten later in the block before they're
used. Very common in the case of srawix and friends.
JIT64: Optimize single float output duplication (PS1 tracking)

Single-precision floating point operations on the Gekko CPU duplicate their
result into the high and low halves of the output register. This is an extra
instruction on x86, and can be avoided if the top half never actually gets used.

This change tracks the usage of the top half of those floating point registers
and avoids the duplication wherever possible.