[llvm] Constant Fold Logf128 calls (PR #84501)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The primary reason we haven't continued to expand float constant-folding is that we don't really want to use the host's libm for target calculations.  Ideally, we want the compiler to be portable: we want to consistently produce the same results.  So we want to use our own library routines.

We have ConstantFoldFP which already uses the host libc, but we don't really want to expand that usage.  gcc uses MFPR, but LLVM can't due to licensing issues.  Maybe we can borrow code from llvm-libc?

https://github.com/llvm/llvm-project/pull/84501