[clang] [compiler-rt] [llvm] [ConstantFolding] Canonicalize constexpr GEPs to i8 (PR #89872)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

btw we're still looking into a performance regression caused by #68882 that still repros with LLVM head, even after the SROA enhancements. this patch looks good, but can we hold off a bit on submitting this?

https://github.com/llvm/llvm-project/pull/89872