[SimplifyLibCalls] Remove over-eager early return in strlen optzns.

Currently we bail out early for strlen calls with a GEP operand, if none
of the GEP specific optimizations fire. But there could be later
optimizations that still apply,  which we currently miss out on.

An example is that we do not apply the following optimization
   strlen(x) == 0 --> *x == 0

Unless I am missing something, there seems to be no reason for bailing
out early there.

Fixes PR47149.

Reviewed By: lebedev.ri, xbolva00

Differential Revision: https://reviews.llvm.org/D85886