The number of registers used for passing f64 in some cases is context
dependent, and thus getNumRegistersForCallingConv is sometimes
inaccurate. For f64, it reports 1 but is sometimes split into 2 32-bit
For GlobalISel, the generic argument assignment code expects
getNumRegistersForCallingConv to return an accurate answer. Switch to
marking these arguments as custom so we can deal with this case as a
custom assignment rather.
This temporarily breaks a few globalisel tests which are fixed by a
future change to use more of the generic infrastructure.