* store immediate values as int64_t, not int. come on, we should be happy

when there are immediates, let's not worry about the memory overhead of
this :)

* add addU64Imm(uint64_t val) to machineinstrbuilder

(seriously: this seems required to support 64-bit immediates cleanly. if it
_really_ gets on your nerves, feel free to pull it out ;) )

coming up next week: "all your floating point constants are belong to us"

llvm-svn: 21208