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

[via llvm-commits]

AtariDreams wrote:

> This issue came up with our downstream backend, but the problematic examples are passing with AArch64. Not easy to create a test for this, but on the other hand the change is fairly self evident.

There is a failing unit test. Maybe that can give an idea of how this changes things.

https://github.com/llvm/llvm-project/pull/87927