McARM: Use a consistent marker for not-set OffsetRegNum.

llvm-svn: 123736
diff --git a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index a34fdb6..7125e98 100644
--- a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -763,7 +763,7 @@
     E = Tok.getLoc();
     Parser.Lex(); // Eat right bracket token.
 
-    int OffsetRegNum = 0;
+    int OffsetRegNum = -1;
     bool OffsetRegShifted = false;
     enum ShiftType ShiftType = Lsl;
     const MCExpr *ShiftAmount = 0;