ART: Rework TargetReg(symbolic_reg, wide)

Make the standard implementation in Mir2Lir and the specialized one
in the x86 backend return a pair when wide = "true". Introduce
WideKind enumeration to improve code readability. Simplify generic
code based on this implementation.

Change-Id: I670d45aa2572eedfdc77ac763e6486c83f8e26b4
diff --git a/compiler/dex/compiler_enums.h b/compiler/dex/compiler_enums.h
index bdedadb..69adb35 100644
--- a/compiler/dex/compiler_enums.h
+++ b/compiler/dex/compiler_enums.h
@@ -548,6 +548,14 @@
 
 std::ostream& operator<<(std::ostream& os, const VolatileKind& kind);
 
+enum WideKind {
+  kNotWide,      // Non-wide view
+  kWide,         // Wide view
+  kRef           // Ref width
+};
+
+std::ostream& operator<<(std::ostream& os, const WideKind& kind);
+
 }  // namespace art
 
 #endif  // ART_COMPILER_DEX_COMPILER_ENUMS_H_