Use consistent compatibility checks in vectorizable_shift

Use consistent compatibility checks in vectorizable_shift

The validation phase of vectorizable_shift used TYPE_MODE to check
whether the shift amount vector was compatible with the shifted vector:

      if ((op1_vectype == NULL_TREE
	   || TYPE_MODE (op1_vectype) != TYPE_MODE (vectype))
 	  && (!slp_node
 	      || SLP_TREE_DEF_TYPE
 		   (SLP_TREE_CHILDREN (slp_node)[1]) != vect_constant_def))

But the generation phase was stricter and required the element types to
be equivalent:

		   && !useless_type_conversion_p (TREE_TYPE (vectype),
						  TREE_TYPE (op1)))

This difference led to an ICE with a later patch.

The first condition seems a bit too lax given that the function
supports vect_worthwhile_without_simd_p, where two different vector
types could have the same integer mode.  But it seems too strict
to reject signed shifts by unsigned amounts or unsigned shifts by
signed amounts; verify_gimple_assign_binary is happy with those.

This patch therefore goes for a middle ground of checking both TYPE_MODE
and TYPE_VECTOR_SUBPARTS, using the same condition in both places.

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_shift): Check the number
	of vector elements as well as the type mode when deciding
	whether an op1_vectype is compatible.  Reuse the result of
	this check when generating vector statements.

From-SVN: r278235