[mips][FastISel] Instantiate the MipsFastISel class only for targets that support FastISel.
Instead of instantiating the MipsFastISel class and checking if the
target is supported in the overriden methods, we should perform that
check before creating the class. This allows us to enable FastISel *only*
for targets that truly support it, ie. MIPS32 to MIPS32R5.
Subscribers: ehostunreach, llvm-commits
Differential Revision: https://reviews.llvm.org/D24824