Having the "same" vector types with different modes means that we can
end up vectorising a constructor with a different mode from the lhs.
This patch adds a VIEW_CONVERT_EXPR in that case.

This showed up on existing tests when testing with fixed-length
-msve-vector-bits=128.

2020-01-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vectorize_slp_instance_root_stmt): Use a
	VIEW_CONVERT_EXPR if the vectorized constructor has a diffeent
	type from the lhs.