[RISCV] Make vsetvli intrinsics default to MA.

[RISCV] Make vsetvli intrinsics default to MA.

The vsetvli insertion pass can replace it with MU if needed by
a using instruction. The vsetvli insertion pass will not convert
MU to MA so we need to start at MA.

Reviewed By: eopXD

Differential Revision: https://reviews.llvm.org/D143790