RISC-V: Bugfix for ICE in block move when zve32f

RISC-V: Bugfix for ICE in block move when zve32f

The exact_div requires the exactly multiple of the divider.
Unfortunately, the condition will be broken when zve32f in
some cases. For example,

potential_ew is 8
BYTES_PER_RISCV_VECTOR * lmul1 is [4, 4]

This patch would like to ensure the precondition of exact_div
when get_vec_mode.

	PR target/112743

gcc/ChangeLog:

	* config/riscv/riscv-string.cc (expand_block_move): Add
	precondition check for exact_div.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112743-1.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>