PEI: Skip dead objects when looking at CSRs

[arsenm]

Summary

On AMDGPU if an SGPR is spilled to a VGPR, the frame index
is deleted. If there were any CSR SGPRs, this woudl
assert when setting the offset.

[arsenm]

ping

[MatzeB]

Seems okay to go ahead with this.
However looking around the code it seems we need similar ifs in various other loops inside the file.

[arsenm]

r301961

lib/CodeGen/PrologEpilogInserter.cpp

for (unsigned i = MinCSFrameIndex; i <= MaxCSFrameIndex; ++i) {
       Offset = alignTo(Offset, Align, Skew);
 
       DEBUG(dbgs() << "alloc FI(" << i << ") at SP[" << -Offset << "]\n");
       MFI.setObjectOffset(i, -Offset);        // Set the computed offset
     }
   } else if (MaxCSFrameIndex >= MinCSFrameIndex) {
     // Be careful about underflow in comparisons agains MinCSFrameIndex.
     for (unsigned i = MaxCSFrameIndex; i != MinCSFrameIndex - 1; --i) {
+      if (MFI.isDeadObjectIndex(i))
+        continue;
+
       unsigned Align = MFI.getObjectAlignment(i);
       // Adjust to alignment boundary
       Offset = alignTo(Offset, Align, Skew);
 
       DEBUG(dbgs() << "alloc FI(" << i << ") at SP[" << Offset << "]\n");
       MFI.setObjectOffset(i, Offset);
       Offset += MFI.getObjectSize(i);
     }