Reapply "[mips][FastISel] Implement shift ops for Mips fast-isel.""

This reapplies r235194, which was reverted in r235495 because it was causing a
failure in our out-of-tree buildbots for MIPS. With the sign-extension patch
in r235718, this patch doesn't cause any problem any more.

llvm-svn: 235878