RISC-V: Fix vid const vector expander for non-npatterns size steps

Prior to this patch the expander would emit vectors like:
{ 0, 0, 5, 5, 10, 10, ...}
as:
{ 0, 0, 2, 2,  4,  4, ...}

This patch sets the step size to the requested value.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector): Fix STEP size in
	expander.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>