[GlobalISel] Fix fewerElementsVectorPhi to insert after G_PHIs (#87927)

Currently the inserted mergelike instructions will be inserted at the
location of the G_PHI. Seems like the behaviour was correct before, but
the rework done in https://reviews.llvm.org/D114198 forgot to include
the part which makes sure the instructions will be inserted after all
the G_PHIs.

(cherry picked from commit 2347020e)