On most cases, the vector type (VTYPE) for the RISC-V Vector extension
is supplied as an immediate value, with either of the VSETVLI or
VSETIVLI instructions. There is however a third instruction VSETVL
which takes the vector type from a general purpose register. That is so
the type can be selected at run-time.

This introduces a macro to load a (valid) vector type into a register.
The syntax follows that of VSETVLI and VSETIVLI, with element size,
group multiplier, then tail and mask policies.