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

llvm-svn: 260427