[X86] Support LEA64_32r in processInstrForSlow3OpLEA and use INC/DEC when possible.

Move the erasing and iterator updating inside to match the
other slow LEA function.

I've adapted code from optTwoAddrLEA and basically rebuilt the
implementation here. We do lose the kill flags now just like
optTwoAddrLEA. This runs late enough in the pipeline that
shouldn't really be a problem.

llvm-svn: 373877