[llvm-stress] Support scalable vectors and out of bounds indices to vector ops

This change does two things.

First, it supports scalable vectors in llvm-stress output.  There's an off-by-default flag -enable-scalable-vectors which generates a scalable vector with 50% probability when a random vector type is selected.  Because of the existing implementation, this tends to skew an entire test towards either scalable or fixed since we most select initial values, not random types.

Second, it selects a random value for the index of an insertelement or extractelement.  As a result, out of bounds indices can not appear for fixed lengh vectors.  Previously, it chose a random constant which was, by construction, in range.