The higher 64 bits of 142[V2DI] is unknown here and it generated incorrect
code when store back to memory. This patch would like to fix this issue
by adding a new SCALAR_MOVE_MERGED_OP for vec_set.
Please note this patch doesn't enable VLS for vec_set, the underlying
patches will support this soon.
gcc/ChangeLog:
* config/riscv/autovec.md: Bugfix.
* config/riscv/riscv-protos.h (SCALAR_MOVE_MERGED_OP): New enum.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/rvv/base/scalar-move-merged-run-1.c: New test.