[llvm] [LSR] Reuse identical casts and instructions across fixups (PR #201839)

[Timur Golubovich via llvm-commits]

timurgol007 wrote:

@nikic, Hi! I've added 4 commits to this PR, each covering a distinct part of the newly added optimization. Would you prefer I split them into separate PRs? (The PR with the first commit is already opened)

https://github.com/llvm/llvm-project/pull/201839