vect: Remove several useless VMAT_INVARIANT checks

In function vectorizable_load, there is one hunk which is
dedicated for the handlings on VMAT_INVARIANT and return
early, it means we shouldn't encounter any cases with
memory_access_type VMAT_INVARIANT in the following code
after that.  This patch is to clean up several useless
checks on VMAT_INVARIANT.  There should be no functional
changes.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_load): Remove some useless checks
	on VMAT_INVARIANT.