[llvm] [InstCombine] Allow moving to larger type if desirable and both from/to types are illegal (PR #171675)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

At least from the diffs of the benchmarks, it seems like the patch may be more beneficial with the original change, so I thought of reverting this to the previous commit. Let you establish whether this is worthwhile to pursue.

https://github.com/llvm/llvm-project/pull/171675