Allow vector types for isplit and iconcat.
Allow vector types for isplit and iconcat.

These two instructions make sense for vector types by simply performing
the same operation on each lane, like most other vector operations.

Problem found by @angusholder's verifier.

stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne