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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123736 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index a34fdb6..7125e98 100644
--- a/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/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;