Add a "get element" instruction for vector registers

[mbitsnbites]

The instruction should transfer a single element from a vector register to a scalar register.

Note that this instruction goes against the regular model of scalar and vector operations since it:

1. Is a single-cycle operation even if it takes a vector register as a source operand (and thus ignores VL).
2. Targets a scalar register even if takes a vector register as a source operand.

As an extension it could copy the vector element to all elements in a destination vector register.