MIPS: Reduce Baker read barrier code size overhead

Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: test-art-target-gtest
Test: testrunner.py --target --optimizing
Test: same tests as above on CI20
Test: booted MIPS32 and MIPS64 in QEMU with poisoning
      in configurations:
      - with Baker read barrier thunks
      - without Baker read barrier thunks
      - ART_READ_BARRIER_TYPE=TABLELOOKUP

Change-Id: I79f320bf8862a04215c76cfeff3118ebc87f7ef2
diff --git a/compiler/optimizing/code_generator_mips64.h b/compiler/optimizing/code_generator_mips64.h
index c94cc93..d03a9ea 100644
--- a/compiler/optimizing/code_generator_mips64.h
+++ b/compiler/optimizing/code_generator_mips64.h
@@ -281,7 +281,8 @@
                                Location root,
                                GpuRegister obj,
                                uint32_t offset,
-                               ReadBarrierOption read_barrier_option);
+                               ReadBarrierOption read_barrier_option,
+                               Mips64Label* label_low = nullptr);
 
   void GenerateTestAndBranch(HInstruction* instruction,
                              size_t condition_input_index,
@@ -592,7 +593,7 @@
 
   void EmitPcRelativeAddressPlaceholderHigh(PcRelativePatchInfo* info_high,
                                             GpuRegister out,
-                                            PcRelativePatchInfo* info_low);
+                                            PcRelativePatchInfo* info_low = nullptr);
 
   void PatchJitRootUse(uint8_t* code,
                        const uint8_t* roots_data,