[SimplifyLibCalls] Don't mark memchr() memory as fully dereferenceable

C11 specifies memchr() as follows:

> The memchr function locates the first occurrence of c (converted
> to an unsigned char) in the initial n characters (each interpreted
> as unsigned char) of the object pointed to by s. The implementation
> shall behave as if it reads the characters sequentially and stops
> as soon as a matching character is found.

In particular, it is well-defined to specify a memchr size larger
than the underlying object, as long as the character is found before
the end of the object.

Differential Revision: https://reviews.llvm.org/D123665