A MidForeign call is used for *unsafe* foreign calls;
a LastForeign call is used for *safe* foreign calls.
Unsafe ones are easy: think of them as a "fat machine instruction".
+In particular, they do *not* kill all live registers (there was a bit
+of code in GHC that conservatively assumed otherwise.)
Safe ones are trickier. A safe foreign call
r = f(x)