Merge pull request #2870 from madewokherd/arginfo

Merge pull request #2870 from madewokherd/arginfo

x86 cleanups for register arguments

Best I can tell, cinfo->args and cfg->args need to be independent before implementing calling conventions where some arguments are in registers, at least if we want to do it cleanly. Also, storing the argument offset in a cfg->args instruction that claims to be for a register seems hacky and confusing to me.