[llvm] DAG: Skip 0 sign handling in minimum/maximum lowering for _ieee case (PR #91326)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LGTM. I don't know if the PowerPC change is correct though.

PPC ISA PDF I found says "The minimum of +0 and –0 is –0" and "The maximum of +0 and –0 is +0"  so I think this is fine 

https://github.com/llvm/llvm-project/pull/91326