[mips][FastISel] Disable code generation for unsupported targets through FastISel.

Summary:
Previously, we would check whether the target is supported or not, only in
fastSelectInstruction(). This means that 64-bit targets could use FastISel too.
We fix this by checking every overridden method of the FastISel class and
by falling back to SelectionDAG if the target isn't supported. This change
should have been committed along with r243638, but somehow I missed it.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11755

llvm-svn: 243986