[LAA] Fix ICE with scAddExpr in forked pointers

The IR from https://github.com/llvm/llvm-project/issues/57368 results
in an assert firing when trying to create a runtime check for the
forked pointer. One of the forks is fine since it's loop invariant,
but the other is a scAddExpr (containing a scAddRecExpr, so not
invariant) when RtCheck::insert expects a scAddRecExpr.

This is a simple fix to just avoid forks which aren't AddRec or
loop invariant. We can allow it as a forked pointer later with
more work.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D133020