[llvm] [InstSimplify] Defer to ConstantFold in simplifyInstrinsic (PR #205061)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

If this is a bit unwieldy, we can always go back to the first commit, which is much simpler to reason about, and is trivially correct with the same improvements: https://github.com/llvm/llvm-project/pull/205061/changes/4615ad25d09720d0a2272effdfb0859b28ab24ad

https://github.com/llvm/llvm-project/pull/205061