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

[Dávid Ferenc Szabó via llvm-commits]

dfszabo 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.

Yes indeed, the test fix show now nicely the expected behavior. 

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