RISC-V: Make tuple vector not tieable to some modes.

RISC-V: Make tuple vector not tieable to some modes.

This patch makes riscv tuple modes not tieable to non-tuple modes. Without
this patch some unnecessary type conversions may occur, especially when zvl
is specified.
E.g. RVVMF2x4HI and RVVM2DI are tieable in gcc trunk, and when extracting
an inner vector mode RVVMF2HI from RVVMF2x4HI and zvl is specified, it will
be converted to DI, which is not expected. But with same inner modes, e.g.
RVVM1x4QI and RVVM1QI, they should be tieable.

        PR target/124448

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_modes_tieable_p): Make tuple modes
	not tieable to some modes.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr124448.c: New test.

Signed-off-by: wangzicong <wangzicong@masscore.cn>