[VPlan] Move Load/Store checks out of tryToWiden (NFC).

Handling LoadInst and StoreInst in tryToWiden seems a bit
counter-intuitive, as there is only an assertion for them and in no
case VPWidenRefipes are created for them.

I think it makes sense to move the assertion to handleReplication, where
the non-widened loads and store are handled.

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D77972