[mips][microMIPS] Implement NOP aliases

This patch implements microMIPS 16-bit (MOVE16 $0, $0) and
32-bit (SLL $0, $0, 0) NOP aliases.

http://reviews.llvm.org/D6440


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222953 91177308-0d34-0410-b5e6-96231b3b80d8