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/utils/mips/assembler_mips.h b/compiler/utils/mips/assembler_mips.h
index 33803bb..e42bb3f 100644
--- a/compiler/utils/mips/assembler_mips.h
+++ b/compiler/utils/mips/assembler_mips.h
@@ -280,6 +280,7 @@
   void Lwpc(Register rs, uint32_t imm19);  // R6
   void Lui(Register rt, uint16_t imm16);
   void Aui(Register rt, Register rs, uint16_t imm16);  // R6
+  void AddUpper(Register rt, Register rs, uint16_t imm16, Register tmp = AT);
   void Sync(uint32_t stype);
   void Mfhi(Register rd);  // R2
   void Mflo(Register rd);  // R2