Create helper class for DWARF expressions.

This ensures we generate valid DWARF opcodes and
it slightly simplifies the debug info writer.

Change-Id: I2ef8a9222c9a129c58f500741acd209b67e2ecff
diff --git a/compiler/dwarf/debug_frame_opcode_writer.h b/compiler/dwarf/debug_frame_opcode_writer.h
index 5a99641..f74f37c 100644
--- a/compiler/dwarf/debug_frame_opcode_writer.h
+++ b/compiler/dwarf/debug_frame_opcode_writer.h
@@ -248,7 +248,7 @@
     }
   }
 
-  void ALWAYS_INLINE DefCFAExpression(void * expr, int expr_size) {
+  void ALWAYS_INLINE DefCFAExpression(uint8_t* expr, int expr_size) {
     if (UNLIKELY(enabled_)) {
       ImplicitlyAdvancePC();
       uses_dwarf3_features_ = true;
@@ -258,7 +258,7 @@
     }
   }
 
-  void ALWAYS_INLINE Expression(Reg reg, void * expr, int expr_size) {
+  void ALWAYS_INLINE Expression(Reg reg, uint8_t* expr, int expr_size) {
     if (UNLIKELY(enabled_)) {
       ImplicitlyAdvancePC();
       uses_dwarf3_features_ = true;
@@ -269,7 +269,7 @@
     }
   }
 
-  void ALWAYS_INLINE ValExpression(Reg reg, void * expr, int expr_size) {
+  void ALWAYS_INLINE ValExpression(Reg reg, uint8_t* expr, int expr_size) {
     if (UNLIKELY(enabled_)) {
       ImplicitlyAdvancePC();
       uses_dwarf3_features_ = true;