[MLton] Shortening dependency chains in the x86 codegen

[Florian Weimer]

* Vesa Karvonen:

> Take a look at the following code for 32-bit endianness swapping
> generated by MLton on x86:

I'm sure that nowaydays, you could just use bswapl.  It's available in
i486 and up, and AFAIK, call compatibles in use today implement it.