Consider building nodes from scalars in vect_slp_analyze_node_operations

Consider building nodes from scalars in vect_slp_analyze_node_operations

If the statements in an SLP node aren't similar enough to be vectorised,
or aren't something the vectoriser has code to handle, the BB vectoriser
tries building the vector from scalars instead.  This patch does the
same thing if we're able to build a viable-looking tree but fail later
during the analysis phase, e.g. because the target doesn't support a
particular vector operation.

This is needed to avoid regressions with a later patch.

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

gcc/
	* tree-vect-slp.c (vect_contains_pattern_stmt_p): New function.
	(vect_slp_convert_to_external): Likewise.
	(vect_slp_analyze_node_operations): If analysis fails, try building
	the node from scalars instead.

gcc/testsuite/
	* gcc.dg/vect/bb-slp-div-2.c: New test.

From-SVN: r278246