JIT64: optimize CA calculations

**JIT64: optimize CA calculations**

Omit carry calculations that get overwritten later in the block before they're used. Very common in the case of srawix and friends.

I previously had a patch to do PS1 optimizations too here but I dropped it for now because it was becoming rather scary and messy (and still broken).

**Please only review the last patch in this PR; the others are just patches that it depends on (the FPRF branch and integeropts branch).**