Micro optimizations to x86 jit mem when fastmem is off

I wanted to play around with jit a bit, so I picked this since it seemed simple.  It doesn't really provide a huge perf boost or anything, so the code complexity might not be worth it.

If this doesn't look right at all, please don't merge.  I won't feel bad at all.

This:
 * Removes the overhead for sw/lw updating the pc, reshifting the opcode, etc. when fastmem is off by going more directly to the mem.
 * Implements lhu/sh in jit (they don't run as much, but they were the next most popular variants in that function for me.)

I'm assuming this is correct for movw:

```
MOV(16, gpr.R(rt), MDisp(EAX, (u32)Memory::base + offset));
```

But I'm not sure, since I tried the same for lbu and it did not work properly (even if I used MOV 32 / AND 32, not sure why...)  That said, everything seems stable with fastmem on or off and lhu/sh only, 32 and 64 bit.

-[Unknown]