Rewrite SLP induction vectorization

Rewrite SLP induction vectorization

This rewrites SLP induction vectorization to handle different
inductions in the different SLP lanes.  It also changes SLP
build to represent the initial value (but not the cycle) so
it can be enhanced to handle outer loop vectorization later.

Note this FAILs gcc.dg/vect/costmodel/x86_64/costmodel-pr30843.c
because it removes one CSE optimization that no longer works
with non-uniform initial value and step.  I'll see to recover
from this after outer loop vectorization of inductions works.

It might be a bit friendlier to variable-size vectors now
but then we're now building the step vector from scalars ...

2020-11-02  Richard Biener  <rguenther@suse.de>

	* tree.h (build_real_from_wide): Declare.
	* tree.c (build_real_from_wide): New function.
	* tree-vect-slp.c (vect_build_slp_tree_2): Remove
	restriction on induction vectorization, represent
	the initial value.
	* tree-vect-loop.c (vect_model_induction_cost): Inline ...
	(vectorizable_induction): ... here.  Rewrite SLP
	code generation.

	* gcc.dg/vect/slp-49.c: New testcase.