[llvm] GlobalISel: Use LibcallLoweringInfo analysis in legalizer (PR #170328)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd commented:

Might be nice to split out some of the reformatting into its own PR. The adjustment in style makes it harder to see what actually is changing.

I wonder if it is better to rename `Helper` to `LibCallGenerator` for the instances at least. That makes the call sites better as `Helper` is quite generic.

https://github.com/llvm/llvm-project/pull/170328