[IR] Use iteration limit in stripPointerCastsAndOffsets (#190472)
Using a SmallPtrSet is not quite free for such a frequently called
operation. However, calls on ill-formed IR are not particularly rare, so
some iteration limit is needed. Therefore, use a simple counter.
Termination statistics on a Clang Release build for N>5:
2448 N=6
1295 N=7
480 N=8
294 N=9
160 N=10
14350 (endless loop)
Therefore, bound the number of iterations by 12, which should cover most
practically relevant cases.
It is worth noting that _all_ of the endless loop cases have the
following form:
%incdec.ptr = getelementptr inbounds nuw i8, ptr %incdec.ptr, i64 <N>
I haven't investigated where exactly this comes from, though; but it
occurred frequent enough to add a special case for this.
