[LegalizeDAG] Use getMemBasePlusOffset to simplify some code. Use other...

[LegalizeDAG] Use getMemBasePlusOffset to simplify some code. Use other signature of getMemBasePlusOffset in another location. NFCI

The code was calculating an offset from a stack pointer SDValue.
This is exactly what getMemBasePlusOffset does. I also replaced
sizeof(int) with a hardcoded 4. We know the type we're operating
on is 4 bytes. But the size of int that the source code is being
compiled with isn't guaranteed to be 4 bytes.

While here replace another use of getMemBasePlusOffset that was
proceeded with a call to getConstant with the other signature
that call getConstant internally.