[VPlan] Handle invariant GEPs in isUniformAfterVectorization.

This fixes a crash caused by legal treating a scalable GEP as invariant,
but isUniformAfterVectorization does not handle GEPs.

Partially fixes https://github.com/llvm/llvm-project/issues/60831.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D144434