vect: Restore optab_vector argument [PR106250]

vect: Restore optab_vector argument [PR106250]

In g:76c3041b856cb0 I'd removed a "C ? optab_vector : optab_mixed_sign"
argument from a call to directly_supported_p, thinking that the argument
only existed because of the condition (which I was removing).  But the
difference between the scalar and vector forms matters for shifts,
so we do still need the argument.

gcc/
	PR tree-optimization/106250
	* tree-vect-loop.cc (vectorizable_reduction): Reinstate final
	argument to directly_supported_p.