[SLC] Optimize mempcpy_chk to mempcpy

As reported in PR46735:

void* f(void *d, const void *s, size_t l)
{
    return __builtin___mempcpy_chk(d, s, l, __builtin_object_size(d, 0));
}

This can be optimized to `return mempcpy(d, s, l);`.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D86019