[LV][NFC] Use ElementCount for getMaxInterleaveFactor

In order to allow targets to disable interleaving for scalable vectors, pass the entire VF's ElementCount to getMaxInterleaveFactor.
This is based off of the approach used here: https://repo.hca.bsc.es/gitlab/rferrer/llvm-epi/-/commit/8d36708507b3c378078b9fe364bc548354aaec86

The plan would then be to disable interleaving on scalable VFs on RISC-V in a follow up patch.
See https://reviews.llvm.org/D143723#4132349

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D144474