[AArch64][GlobalISel] Add heuristics for G_FCONSTANT localization.

Now that in an earlier commit we adopt the heuristics for SDAG's expansion
of 32/64b fpimms to either GPR materializations or CP load, we can also improve
the localizer to also understand the same heuristics. This avoids localizing
expensive immediates as that increases code size.

The combination of these two changes results in minor improvements in CTMark -Os,
and bigger improvements in some other cases.