RISC-V: Add more vector-vector extract cases.

This adds a V16SI -> V4SI and related i.e. "quartering" vector-vector
extract expander for VLS modes.  It helps with spills in x264 that may
cause a load-hit-store.

gcc/ChangeLog:

	* config/riscv/autovec.md (vec_extract<mode><vls_quarter>):
	Add quarter vec-vec extract.
	* config/riscv/vector-iterators.md: New iterators.