[mips][mips64r6] ssnop is deprecated on MIPS32r6/MIPS64r6

Summary: Depends on D4120

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: zoran.jovanovic, vmedic

Differential Revision: http://reviews.llvm.org/D4121

llvm-svn: 211021