tree-optimization/115538 - possible wrong-code with SLP conversion

tree-optimization/115538 - possible wrong-code with SLP conversion

The following fixes a latent issue where we use ranges to verify
correctness of a vector conversion optimization.  We rely on ranges
from 'op0' which for SLP is extracted from the representative stmt
which does not necessarily correspond to any actual scalar operation.
We also do not verify the range of all scalar lanes in the SLP
operand match.  The following rectifies this, restricting the support
to single-lane SLP nodes at this point - on branches we'd simply
not perform this optimization with SLP.

	PR tree-optimization/115538
	* tree-vectorizer.h (vect_get_slp_scalar_def): Declare.
	* tree-vect-slp.cc (vect_get_slp_scalar_def): New helper.
	* tree-vect-generic.cc (expand_vector_conversion): Adjust.
	* tree-vect-stmts.cc (vectorizable_conversion): For SLP
	correctly look at ranges of the scalar defs of the SLP operand.
	(supportable_indirect_convert_operation): Likewise.