[PATCH] [FastISel][X86] Assign the correct register class to folded operations

[Keno Fischer]

I looked at implementing this in foldMemoryOperandImpl, but it seems like right now it can only return one instruction and does not insert it, which doesn't work if we want to insert the COPY for constraining the register class. 

I did see a FIXME that says
```
// FIXME: change foldMemoryOperandImpl semantics to also insert NewMI.
```
so maybe I should just try that?

http://reviews.llvm.org/D6262