[AMDGPU] Increases available SGPR for Calling Convention

Summary:
SGPR in CC can be either hw initialized or set by other chained shaders
and so this increases the SGPR count availalbe to CC to 105.

Change-Id: I3dfadc750fe4a3e2bd07117a2899fd13f3e2fef3

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61261

llvm-svn: 360778