[Mips][FastISel] Fix handling of icmp with i1 type

The Mips FastISel back-end does not extend i1 values while lowering icmp.
Ensure that we bail into DAG ISel when handling this case.

Patch by Dragan Mladjenovic.

Differential Revision: https://reviews.llvm.org/D49290

llvm-svn: 337288