[llvm] DAG: Use phi to create vregs instead of the constant input (PR #129464)

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

This commit looks to be the source of a miscompile. 
[129464-reproducer.zip](https://github.com/user-attachments/files/19236058/129464-reproducer.zip) has the input IR, a Makefile, and a test program that shows the incorrect results.

To quote the README
> I know the code is bad - generating a less terribly inefficient routine is parallel work.
> 
> The --disable-verify in the Makefile is due to an unrelated issue where llvm.masked.load.*.p7 doesn't get erased by buffer fat pointer lowering because/even though it's dead by the timet that pass is reached.
> 
> I don't have a good theory of what's going on here.

I might have a smaller example later, but I thought I'd flag this in the meantime

https://github.com/llvm/llvm-project/pull/129464