tree-optimization/104114 - avoid diagnosing V1mode lowering

tree-optimization/104114 - avoid diagnosing V1mode lowering

Currently we diagnose vector lowering of V1mode operations that
are not natively supported into V_C_E, scalar op plus CTOR with
-Wvector-operation-performance but that's hardly useful behavior
even though the way we lower things can be improved.

The following disables the diagnostics for the cases the vect.exp
testsuite runs into, on x86 that are vect-cond-11.c and
vect-singleton_1.c.

2022-01-19  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104114
	* tree-vect-generic.cc (expand_vector_piecewise): Do not diagnose
	single element vector decomposition.