[llvm] [AggressiveInstCombine] Fold i64 x i64 -> i128 multiply-by-parts (PR #156879)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Hi. Could we try splitting one of these into a separate patch, perhaps one of the high variants, to split this up a little?

It looks like having a phase-ordering test would be useful too, to protect against codegen changing in the future.

https://github.com/llvm/llvm-project/pull/156879