MIPS: Support swaps between 128-bit locations

Add support for swaps between two SIMDStackSlots, two
VectorRegisters (extended FpuRegister) and between a
SIMDStackSlot and a VectorRegister.

This fixes test 623-checker-loop-regressions for
MIPS64R6 and MIPS32R6.

Test: ./testrunner.py --optimizing --target in QEMU (MIPS64R6)
Test: ./testrunner.py --optimizing --target in QEMU (MIPS32R6)

Change-Id: I36aa209f79790fb6c08b9a171f810769a6b40afc
diff --git a/compiler/optimizing/code_generator_mips.h b/compiler/optimizing/code_generator_mips.h
index 7845e31..cf8e7a3 100644
--- a/compiler/optimizing/code_generator_mips.h
+++ b/compiler/optimizing/code_generator_mips.h
@@ -145,6 +145,7 @@
   void RestoreScratch(int reg) OVERRIDE;
 
   void Exchange(int index1, int index2, bool double_slot);
+  void ExchangeQuadSlots(int index1, int index2);
 
   MipsAssembler* GetAssembler() const;