[llvm] [Transforms] Mirror optimizeStrRChr with optimizeStrChr (PR #77685)

[via llvm-commits]

AtariDreams wrote:

@MaskRay So the big question is: why would we return a poison value for strrchr but not strchr? why not let them have the same undefined behavior, or defined behavior (since we are allowed by the standard to have behavior for undefined behavior). 

https://github.com/llvm/llvm-project/pull/77685