[mips][FastISel] Specify which types we handle for integer extension.

Summary:
Perform integer extension only when the destination type is one of
i8, i16 & i32 and when the source type is i1, i8 or i16. For other
combinations we fall back to SelectionDAG.

This fixes the test MultiSource/Benchmarks/7zip that was failing in our
out-of-tree MIPS buildbots.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9243

llvm-svn: 235718