vect: Enhance condition check to use partial vectors

vect: Enhance condition check to use partial vectors

This patch is derived from the review of vector with length patch
series.  The length-based partial vector approach doesn't support
reduction so far, so we would like to disable vectorization with
partial vectors explicitly for it in vectorizable_condition.
Otherwise, it will cause some unexpected failures for a few cases
like gcc.dg/vect/pr65947-2.c.

But if we disable it for the cases excepting for reduction_type equal
to EXTRACT_LAST_REDUCTION, it cause one regression failure on aarch64:

  gcc.target/aarch64/sve/reduc_8.c -march=armv8.2-a+sve

The disabling makes the outer loop can't work with partial vectors,
the check fails.  But the case is safe to adopt it.  As Richard S.
pointed out in the review comments, the extra inactive lanes only
matter for double reductions, so this patch is to permit vectorization
with partial vectors for cases EXTRACT_LAST_REDUCTION or nested-cycle
reduction.

Bootstrapped/regtested on aarch64-linux-gnu.

gcc/ChangeLog:

	* tree-vect-stmts.c (vectorizable_condition): Prohibit vectorization
	with partial vectors explicitly excepting for EXTRACT_LAST_REDUCTION
	or nested-cycle reduction.