[GMR] Further improve the FunctionInfo API inside of GlobalsModRef, NFC.

This takes the operation of merging a callee's information into the
current information and embeds it into the FunctionInfo type itself.
This is much cleaner as now we don't need to expose iteration of the
globals, etc.

Also, switched all the uses of a raw integer two maintain the mod/ref
info during the SCC walk into just directly manipulating it in the
FunctionInfo object.

llvm-svn: 242976