RISC-V: Fix uninitialized and redundant use of which_alternative

When pass split2 starts, which_alternative is random depending on
last set of certain pass.

Even initialized, the generated movement is redundant.
The movement can be generated by assembly output template.

Signed-off-by: demin.han <demin.han@starfivetech.com>

gcc/ChangeLog:

	* config/riscv/autovec.md: Delete which_alternative use in split

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/madd-split2-1.c: New test.
Signed-off-by: demin.han <demin.han@starfivetech.com>