MIPS32: Refactor implicit null checks in array/field get/set.

Rationale: on MIPS32 64-bit loads and stores may be performed
as pairs of 32-bit loads/stores. Implicit null checks must be
associated with the first 32-bit load/store in a pair and not
the last. This change ensures proper association of said checks
(a few were done after the last 32-bit load/store in a pair)
and lays ground for further improvements in array/field get/set.

Test: booted MIPS32 in QEMU
Test: test-art-host-gtest
Test: test-art-target-run-test-optimizing in QEMU

Change-Id: I3674947c00bb17930790a7a47c9b7aadc0c030b8
diff --git a/compiler/optimizing/code_generator_mips.h b/compiler/optimizing/code_generator_mips.h
index 63a0345..46810d6 100644
--- a/compiler/optimizing/code_generator_mips.h
+++ b/compiler/optimizing/code_generator_mips.h
@@ -257,6 +257,7 @@
   void GenerateDivRemWithAnyConstant(HBinaryOperation* instruction);
   void GenerateDivRemIntegral(HBinaryOperation* instruction);
   void HandleGoto(HInstruction* got, HBasicBlock* successor);
+  auto GetImplicitNullChecker(HInstruction* instruction);
 
   MipsAssembler* const assembler_;
   CodeGeneratorMIPS* const codegen_;