[Attributor] Convert test to opaque pointers (NFC)

This converts the arg-count-mismatch.ll test to opaque pointers.
The bitcasts of the called functions are now implicit and this
affects behavior: We now infer memory attributes. This should be
fine, as function-level attributes are not affected by signature
mismatches.

Differential Revision: https://reviews.llvm.org/D153406