vect: Check partial vector param for supports_partial_vectors [PR104015]

vect: Check partial vector param for supports_partial_vectors [PR104015]

As described in PR104015, the function partial_vectors_supported_p
mainly checks optabs for partial vectors support query, but we
still have one parameter param_vect_partial_vector_usage to control
the capability.

Power9 introduces vector with length instructions (for
len_load/len_store) but we don't enable partial vector on it by
default. It should be considered as not supporting partial vector by
default. This fix is to make the flag supports_partial_vectors check
param_vect_partial_vector_usage accordingly.

gcc/ChangeLog:

	PR tree-optimization/104015
	* tree-vect-loop.c (vect_analyze_loop): Check
	param_vect_partial_vector_usage for supports_partial_vectors.