Merging r225529:

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r225529 | dsanders | 2015-01-09 17:21:30 +0000 (Fri, 09 Jan 2015) | 18 lines

[mips] Add support for accessing $gp as a named register.

Summary:
Mips Linux uses $gp to hold a pointer to thread info structure and accesses it
with a named register. This makes this work for LLVM.

The N32 ABI doesn't quite work yet since the frontend generates incorrect IR
for this case. It neglects to truncate the 64-bit GPR to a 32-bit value before
converting to a pointer. Given correct IR (as in the testcase in this patch),
it works correctly.

Reviewers: sstankovic, vmedic, atanasyan

Reviewed By: atanasyan

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6893
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llvm-svn: 231466