Merging r260427:

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r260427 | nha | 2016-02-10 12:13:58 -0800 (Wed, 10 Feb 2016) | 16 lines

AMDGPU: Release the scavenged offset register during VGPR spill

Summary:
This fixes a crash where subsequent spills would be unable to scavenge
a register. In particular, it fixes a crash in piglit's
spec@glsl-1.50@execution@geometry@max-input-components (the test still
has a shader that fails to compile because of too many SGPR spills, but
at least it doesn't crash any more).

This is a candidate for the release branch.

Reviewers: arsenm, tstellarAMD

Subscribers: qcolombet, arsenm

Differential Revision: http://reviews.llvm.org/D16558
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_38@260687 91177308-0d34-0410-b5e6-96231b3b80d8