SwitchInt-terminated basic block to see if the condition of that
an enum discriminant.
We expect such blocks to have a call to
discriminant as their last statement like so:
... _42 = discriminant(_1) SwitchInt(_42, ..)
If the basic block matches this pattern, this function returns the place corresponding to the
_1 in the example above) as well as the
AdtDef of that enum.