At the moment, we create insertelement instructions directly after
LastInst when inserting scalar values in a vector in
This results in invalid IR when LastInst is a phi, followed by another
phi. In that case, the new instructions should be inserted just after
the last PHI node in the block.
At the moment, I don't think the problematic case can be triggered, but
it can happen once predicate regions are merged and multiple
VPredInstPHI recipes are in the same block (D100260).