AMDGPU: Check MRI for callee saved regs instead of TRI

This should the same, but MRI does allow dynamically changing the CSR
set, although currently not used.

llvm-svn: 364425
diff --git a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
index 5872f20..d6c93fd 100644
--- a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
@@ -250,7 +250,7 @@
 
   int NumLanes = Size / 4;
 
-  const MCPhysReg *CSRegs = TRI->getCalleeSavedRegs(&MF);
+  const MCPhysReg *CSRegs = MRI.getCalleeSavedRegs();
 
   // Make sure to handle the case where a wide SGPR spill may span between two
   // VGPRs.