[llvm] Reland "Redesign Straight-Line Strength Reduction (SLSR) (#162930)" (PR #169614)

[Fei Peng via llvm-commits]

fiigii wrote:

This commit (https://github.com/llvm/llvm-project/pull/169614/commits/6519805f5c845a071af66748280145c87eefe45f) solves the correctness issue on top of the original change. It calls `SE->canReuseInstruction` to check if reusing an instruction may introduce more poison than its SCEV. The commit also does a little refactoring to avoid calling canReuseInstruction at multiple positions. 

https://github.com/llvm/llvm-project/pull/169614