[mips] Split expandLoadImmReal into multiple methods. NFC

The `expandLoadImmReal` handles four different and almost non-overlapping
cases: loading a "single" float immediate into a GPR, loading a "single"
float immediate into a FPR, and the same couple for a "double" float
immediate.

It's better to move each `else if` branch into separate methods.

llvm-svn: 374164