vect: Allow signed types in average fallback [PR89007]

The average fallback is restricted to unsigned types, but it also works for
signed types.  Arithmetic right shifts round down, and the low-bit carry
adjusts the result in the same way.

Remove the unsigned restriction.  Use target_supports_op_p rather than
optab_for_tree_code to check whether the fallback operations are available.

gcc/ChangeLog:

	PR tree-optimization/89007
	* tree-vect-patterns.cc (vect_recog_average_pattern): Allow signed
	types in the fallback.  Check whether the fallback operations are
	supported.

gcc/testsuite/ChangeLog:

	PR tree-optimization/89007
	* gcc.target/i386/pr89007.c: New test.