RISC-V: Fix single-step expansion [PR126550].

In expand_const_vector_single_step_npatterns, we use a second vector
builder that is derived from the main one, in order to create a
different alternating pattern for constant synthesis.  We later use the
second builder's number of patterns for creating an intermediate
operation.  The second builder can have a different number of patterns
than the first one, however, for example when its "global" pattern is
simpler.

This patch uses npatterns from the original builder.

	PR target/126550

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector_single_step_npatterns):
	User global builder's npatterns.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr126550-2.c: New test.
	* gcc.target/riscv/rvv/autovec/pr126550.c: New test.

(cherry picked from commit 7cdbbbdecb09e518e28c5116b05e0018b9460b95)