constant switch test bug in x86Translate

[Matthew Fluet]

> More progress, but not there yet.  Here's the new error.
> 
>    x86 code gen starting
>       outputC starting
>       outputC finished in 1.720
>       outputAssembly starting
> 	 translateChunk raised
>       outputAssembly raised in 354.570
>    x86 code gen raised in 551.120
> Compile SML raised in 1060.450
> mlton: toX86Operand: strange Offset

I'm looking into it, but are you reasonable confident that the program at
this point is valid?  Essentially, the error above is arising when I try
to translate an 
MachineOutput.Operand.Offset {base, offset, ty}
but the base turned out to be an immediate value (i.e. a constant int,
char, word, or label).  I can translate it into something, but it doesn't
seem to be a reference to a location on the heap.