RISC-V: Add vec_extract for BI -> QI.

RISC-V: Add vec_extract for BI -> QI.

This patch adds a vec_extract expander that extracts a QImode from a
vector mask mode.  In doing so, it helps recognize a "live
operation"/extract last idiom for mask modes.  It fixes the ICE in
tree-vect-live-6.c by circumventing the fallback code in
extract_bit_field_1.  The problem there is still latent, though, and
needs to be addressed separately.

gcc/ChangeLog:

	* config/riscv/autovec.md (vec_extract<mode>qi): New expander.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/live-2.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/live_run-2.c: New test.