RISC-V: Fix can_find_related_mode_p for VLS types

RISC-V: Fix can_find_related_mode_p for VLS types

can_find_related_mode_p incorrectly handled VLS (Vector Length Specific)
types by using TARGET_MIN_VLEN directly, which is in bits, instead of
converting it to bytes as required.

This patch fixes the issue by dividing TARGET_MIN_VLEN by 8 to convert
from bits to bytes when calculating the number of units for VLS modes.

The fix enables proper vectorization for several test cases:
- zve32f-1.c: Now correctly finds vector mode for SF mode in foo3,
  enabling vectorization of an additional loop.
- zve32f_zvl256b-1.c and zve32x_zvl256b-1.c: Added -mrvv-max-lmul=m2
  option to handle V8SI[2] (vector array mode) requirements during
  vectorizer analysis, which needs V16SI to pass, and V16SI was enabled
  incorrectly before.

Changes since V4:
- Fix testsuite, also triaged why changed.

gcc/ChangeLog:

	* config/riscv/riscv-selftests.cc (riscv_run_selftests): Call
	run_vectorize_related_mode_selftests.
	(test_vectorize_related_mode): New function to test
	vectorize_related_mode behavior.
	(run_vectorize_related_mode_selftests): New function to run all
	vectorize_related_mode tests.
	(run_vectorize_related_mode_vla_selftests): New function to test
	VLA modes.
	(run_vectorize_related_mode_vls_rv64gcv_selftests): New function to
	test VLS modes on rv64gcv.
	(run_vectorize_related_mode_vls_rv32gc_zve32x_zvl256b_selftests):
	New function to test VLS modes on rv32gc_zve32x_zvl256b.
	(run_vectorize_related_mode_vls_selftests): New function to run all
	VLS mode tests.
	* config/riscv/riscv-v.cc (can_find_related_mode_p): Fix VLS type
	handling by converting TARGET_MIN_VLEN from bits to bytes.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/zve32f-1.c: Update expected
	vectorization count from 2 to 3.
	* gcc.target/riscv/rvv/autovec/zve32f_zvl256b-1.c: Add
	-mrvv-max-lmul=m2 option.
	* gcc.target/riscv/rvv/autovec/zve32x_zvl256b-1.c: Add
	-mrvv-max-lmul=m2 option.