[llvm] [SelectOpt] Don't convert constant selects to branches. (PR #110858)

[Sotiris Apostolakis via llvm-commits]

sapostolakis wrote:

Makes sense to specialize for constant values but the condition cost also matters.   
With a select the condition is on the critical path, while with a branch computing the condition is not blocking the instruction. 
Enabling it in a target-independent manner makes sense but a bit worried about any potential downsides. 
Was this change motivated by performance diff on some workload/microbenchmark?

https://github.com/llvm/llvm-project/pull/110858