[mips][mips64r6] Add sel.s and sel.d

Summary:
Also use named constants for common opcode fields.

Depends on D3669

Reviewers: jkolek, vmedic, zoran.jovanovic

Differential Revision: http://reviews.llvm.org/D3670

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208582 91177308-0d34-0410-b5e6-96231b3b80d8