[mips] Merge most of the big/little endian checks in atomic.ll

Summary:
There is very little difference between the big and little endian cases in
test/CodeGen/Mips/atomic.ll. Merge them together using multiple
FileCheck prefixes.

Depends on D4117

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4118

llvm-svn: 211013