[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211018 91177308-0d34-0410-b5e6-96231b3b80d8
[mips][mips64r6] cl[oz], and dcl[oz] are re-encoded in MIPS32r6/MIPS64r6

Summary:
There is no change to the restrictions, just the result register is stored
once in the encoding rather than twice. The rt field is zero in
MIPS32r6/MIPS64r6.

Depends on D4119

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4120

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211019 91177308-0d34-0410-b5e6-96231b3b80d8
[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211021 91177308-0d34-0410-b5e6-96231b3b80d8