[Mips][GISel] Fix a couple issues with passing f64 in 32-bit GPRs. (#69131)

MipsIncomingValueHandler::assignCustomValue should return 1 instead of
2. The return value is the number of additional ArgLocs being consumed.
It's assumed that at least 1 is consumed.

Correct the LocVT used for the spill when there are no registers left.
It should be f64 instead of i32. This allows a workaround to be removed
in the SelectionDAG path.