ART: Print short register names with MIPS64

MIPS64 back-end would print register names as "GpuRegister: X" or
"FpuRegister: X". Space separates Checker attributes and therefore
caused a test to fail. This patch changes it to print "aX" and "fX"
respectively.

Change-Id: Iffae989f25563ef074dd7ef0b09cdfe438119cee
diff --git a/compiler/optimizing/code_generator_mips64.cc b/compiler/optimizing/code_generator_mips64.cc
index 4722e42..799827a 100644
--- a/compiler/optimizing/code_generator_mips64.cc
+++ b/compiler/optimizing/code_generator_mips64.cc
@@ -971,11 +971,11 @@
 }
 
 void CodeGeneratorMIPS64::DumpCoreRegister(std::ostream& stream, int reg) const {
-  stream << Mips64ManagedRegister::FromGpuRegister(GpuRegister(reg));
+  stream << GpuRegister(reg);
 }
 
 void CodeGeneratorMIPS64::DumpFloatingPointRegister(std::ostream& stream, int reg) const {
-  stream << Mips64ManagedRegister::FromFpuRegister(FpuRegister(reg));
+  stream << FpuRegister(reg);
 }
 
 void CodeGeneratorMIPS64::InvokeRuntime(QuickEntrypointEnum entrypoint,