[CFIInserter] Fix handling of OpRelOffset.

The variable `SetOffset` holds the offset of CFA from CFA register. The
cfi_rel_offset directive means we should add relative offset to
`SetOffset`.

Author: Mikhail Gudim

llvm/lib/CodeGen/CFIInstrInserter.cpp

@@ -217,7 +217,7 @@ void CFIInstrInserter::calculateOutgoingCFAInfo(MBBCFAInfo &MBBInfo) {
         CSRReg = CFI.getRegister2();
         break;
       case MCCFIInstruction::OpRelOffset:
-        CSROffset = CFI.getOffset() - SetOffset;
+        CSROffset = CFI.getOffset() + SetOffset;
         break;
       case MCCFIInstruction::OpRestore:
         CSRRestored.set(CFI.getRegister());