[mips][ias] EF_MIPS_MICROMIPS should iff microMIPS code was emitted.

Summary:
This fixes PR27682. Additionally, '.set micromips' by itself is not sufficient
to raise the EF_MIPS_MICROMIPS flag. It is also necessary to emit a microMIPS
instruction. This has also been fixed.

Reviewers: sdardis, vkalintiris, rafael

Subscribers: rafael, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20214

llvm-svn: 269639