[mips] Split expandLoadImmReal into multiple methods. NFC
[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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@374164 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm