expmed: Use GET_MODE_PRECISION and expander's output mode.

expmed: Use GET_MODE_PRECISION and expander's output mode.

This changes the vec_extract path of extract_bit_field to use
GET_MODE_PRECISION instead of GET_MODE_BITSIZE and uses
the mode obtained from insn_data[icode].operand[0] as target mode.

Also, it adds a vec_extract<mode>bi expander for riscv that maps
to vec_extract<mode>qi.  This fixes an ICE on riscv where we did
not find a vec_extract optab and continued with the generic code
which requires 1-byte alignment that riscv mask modes do not provide.

Apart from that it adds poly_int support to riscv's vec_extract
expander and makes the RVV..BImode -> QImode expander call
emit_vec_extract in order not to duplicate code.

gcc/ChangeLog:

	PR target/112773

	* config/riscv/autovec.md (vec_extract<mode>bi): New expander
	calling vec_extract<mode>qi.
	* config/riscv/riscv-protos.h (riscv_legitimize_poly_move):
	Export.
	(emit_vec_extract): Change argument from poly_int64 to rtx.
	* config/riscv/riscv-v.cc (shuffle_extract_and_slide1up_patterns):
	Ditto.
	* config/riscv/riscv.cc (riscv_legitimize_poly_move): Export.
	(riscv_legitimize_move): Use rtx instead of poly_int64.
	* expmed.cc (store_bit_field_1): Change BITSIZE to PRECISION.
	(extract_bit_field_1): Change BITSIZE to PRECISION and use
	return mode from insn_data as target mode.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/pr112773.c: New test.