Fail vectorization when not using SLP and --param vect-force-slp == 1

Fail vectorization when not using SLP and --param vect-force-slp == 1

The following adds --param vect-force-slp to enable the transition
to full SLP.  Full SLP is enforced during stmt analysis where it
detects failed SLP discovery and at loop analysis time where it
avoids analyzing a loop with SLP disabled.  Failure to SLP results
in vectorization to fail.

	* params.opt (vect-force-slp): New param, default 0.
	* doc/invoke.texi (--param vect-force-slp): Document.
	* tree-vect-loop.cc (vect_analyze_loop_2): When analyzing
	without SLP but --param vect-force-slp is 1 fail.
	* tree-vect-stmts.cc (vect_analyze_stmt): Fail vectorization
	for non-SLP stmts when --param vect-force-slp is 1.