MMU: simplify code to restore original data register after failed load

Instead of passing the value around constantly, just store it in the regcache,
note where it is, and restore it on the exception path.

This saves a whole bunch of pushing and popping and gives a ~5% speed boost
in Rebel Strike. It's a bit ugly, but it simplifies a lot of code and is
faster, too.

Source/Core/Core/HW/Memmap.h

@@ -96,13 +96,6 @@ u16 Read_U16(const u32 address);
 u32 Read_U32(const u32 address);
 u64 Read_U64(const u32 address);
 
-u32 Read_S8_Val(const u32 address, u32 var);
-u32 Read_U8_Val(const u32 address, u32 var);
-u32 Read_S16_Val(const u32 address, u32 var);
-u32 Read_U16_Val(const u32 address, u32 var);
-u32 Read_U32_Val(const u32 address, u32 var);
-u64 Read_U64_Val(const u32 address, u64 var);
-
 // Useful helper functions, used by ARM JIT
 float Read_F32(const u32 address);
 double Read_F64(const u32 address);