Jit64: Use MOV instead of ADD for updating imm Ra

The old code would first MOV the old imm into Ra and then do an ADD.
We can skip the ADD and just MOV the new imm.

Source/Core/Core/PowerPC/Jit64/Jit_LoadStore.cpp

@@ -505,10 +505,10 @@ void Jit64::stX(UGeckoInstruction inst)
       }
       else
       {
-        RCOpArg Ra = gpr.UseNoImm(a, RCMode::ReadWrite);
+        RCOpArg Ra = gpr.UseNoImm(a, RCMode::Write);
         RegCache::Realize(Ra);
         MemoryExceptionCheck();
-        ADD(32, Ra, Imm32((u32)offset));
+        MOV(32, Ra, Imm32(addr));
       }
     }
   }

Source/Core/Core/PowerPC/Jit64/Jit_LoadStoreFloating.cpp

@@ -144,10 +144,10 @@ void Jit64::stfXXX(UGeckoInstruction inst)
       }
       else
       {
-        RCOpArg Ra = gpr.UseNoImm(a, RCMode::ReadWrite);
+        RCOpArg Ra = gpr.UseNoImm(a, RCMode::Write);
         RegCache::Realize(Ra);
         MemoryExceptionCheck();
-        ADD(32, Ra, Imm32((u32)imm));
+        MOV(32, Ra, Imm32(addr));
       }
     }
     return;