[mips][mips64r6] ll, sc, lld, and scd are re-encoded on MIPS32r6/MIPS64r6.

Summary:
The linked-load, store-conditional operations have been re-encoded such
that have a 9-bit offset instead of the 16-bit offset they have prior to
MIPS32r6/MIPS64r6.

While implementing this, I noticed that the atomic load/store pseudos always
emit a sign extension using sll and sra. I have improved this to use seb/seh
when they are available (MIPS32r2/MIPS64r2 and above).

Depends on D4118

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4119