AMDGPU/SI: Fix spilling of 96-bit registers

Summary:
It seems like this was broken in r252327.  I thought we had test cases
for this, but it's really hard to tirgger spills of this exact register
size since they aren't used very much.

Reviewers: arsenm, nhaehnle

Subscribers: nhaehnle, arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19021

llvm-svn: 266152