[TableGen] Rewrite type set intersection in type inference

The previous code had a bug when dealing with matching iPTR against a
set of integer types. It was trying to handle it all in a compact way,
but that implementation couldn't be modified to correct the problem in
a simple way. The code wasn't long, and it was easier to rewrite it.

The actual issue was that non-scalar-integer types were considered when
matching against iPTR. For example {iPTR} intersected with {i32 f32}
was {iPTR} (due to multiple types in the other set), but should be just
{i32}, because i32 is the only integer scalar in the other set.