vect: Do not try to duplicate_and_interleave one-element mode.

PR112694 shows that we try to create sub-vectors of single-element
vectors because can_duplicate_and_interleave_p returns true.
The problem resurfaced in PR116611.

This patch makes can_duplicate_and_interleave_p return false
if count / nvectors > 0 and removes the corresponding check in the riscv
backend.

This partially gets rid of the FAIL in slp-19a.c.  At least when built
with cost model we don't have LOAD_LANES anymore.  Without cost model,
as in the test suite, we choose a different path and still end up with
LOAD_LANES.

Bootstrapped and regtested on x86 and power10, regtested on
rv64gcv_zvfh_zvbb.  Still waiting for the aarch64 results.

Regards
 Robin

gcc/ChangeLog:

	PR target/112694
	PR target/116611.

	* config/riscv/riscv-v.cc (expand_vec_perm_const): Remove early
	return.
	* tree-vect-slp.cc (can_duplicate_and_interleave_p): Return
	false when we cannot create sub-elements.