[mips] Emit the .insn directive for empty basic blocks.

Summary:
In microMIPS, labels need to know whether they are on code or data. This is
indicated with STO_MIPS_MICROMIPS and can be inferred by being followed
by instructions. For empty basic blocks, we can ensure this by emitting the
.insn directive after the label.

Also, this fixes some failures in our out-of-tree microMIPS buildbots, for the
exception handling regression tests under: SingleSource/Regression/C++/EH

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9530

llvm-svn: 236815