tree-optimization/105175 - avoid -Wvector-operation-performance

tree-optimization/105175 - avoid -Wvector-operation-performance

This avoids -Wvector-operation-performance diagnostics for vectorizer
produced code.  It's unfortunate the warning_at code in
tree-vect-generic.cc needs adjustments but the diagnostic suppression
code doesn't magically suppress those otherwise.

2022-04-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105175
	* tree-vect-stmts.cc (vectorizable_operation): Suppress
	-Wvector-operation-performance if using emulated vectors.
	* tree-vect-generic.cc (expand_vector_piecewise): Do not diagnose
	-Wvector-operation-performance when suppressed.
	(expand_vector_parallel): Likewise.
	(expand_vector_comparison): Likewise.
	(expand_vector_condition): Likewise.
	(lower_vec_perm): Likewise.
	(expand_vector_conversion): Likewise.

	* gcc.dg/pr105175.c: New testcase.