RISC-V: autovec: Verify that GET_MODE_NUNITS is a multiple of 2.

RISC-V: autovec: Verify that GET_MODE_NUNITS is a multiple of 2.

While working on autovectorizing for the RISCV port I encountered an issue
where can_duplicate_and_interleave_p assumes that GET_MODE_NUNITS is a
evenly divisible by two. The RISC-V target has vector modes (e.g. VNx1DImode),
where GET_MODE_NUNITS is equal to one.

Tested on RISCV and x86_64-linux-gnu. Okay?

gcc/
	* tree-vect-slp.cc (can_duplicate_and_interleave_p):
	Check that GET_MODE_NUNITS is a multiple of 2.