[mips][sched] Temporarily rename IIAlu to IIM16Alu. NFC.

Summary:
The only instructions left in IIAlu are MIPS16 specific. We're not
implementing a MIPS16 scheduler at this time so rename the class to make it
obvious that they are MIPS16 instructions.

Reviewers: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12188

llvm-svn: 248267