RISC-V: Add missing define_insn_reservation to tt-ascalon-d8.md [PR121982]

RISC-V: Add missing define_insn_reservation to tt-ascalon-d8.md [PR121982]

The tt-ascalon-d8's pipeline description has reservations for 16-bit, 32-bit
and 64-bit vector integer divides, but was missing a reservation for 8-bit
vector integer divides, leading to an ICE.  Add the missing reservation.

2025-09-22  Peter Bergner  <bergner@tenstorrent.com>

gcc/
	PR target/121982
	* config/riscv/tt-ascalon-d8.md (tt_ascalon_d8_vec_idiv_byte): New
	define_insn_reservation.

gcc/testsuite/
	PR target/121982
	* gcc.target/riscv/pr121982.c: New test.

Signed-off-by: Peter Bergner <bergner@tenstorrent.com>