[TableGen] Add a way of getting the number of generic opcodes without including modular CodeGen headers.

This is a bit of a hack, but removes a cycle that broke modular builds
of LLVM. Of course the cycle is still there in form of a dependency
on the .def file.

llvm-svn: 323383
diff --git a/llvm/utils/TableGen/CodeGenTarget.h b/llvm/utils/TableGen/CodeGenTarget.h
index 7280d70..4d5a0ed 100644
--- a/llvm/utils/TableGen/CodeGenTarget.h
+++ b/llvm/utils/TableGen/CodeGenTarget.h
@@ -140,6 +140,9 @@
     return *I->second;
   }
 
+  /// Returns the number of predefined instructions.
+  static unsigned getNumFixedInstructions();
+
   /// getInstructionsByEnumValue - Return all of the instructions defined by the
   /// target, ordered by their enum value.
   ArrayRef<const CodeGenInstruction *>