[llvm] [IR][AsmParser] Revamp how floating-point literals work in LLVM IR. (PR #121838)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

While working on other changes, I noticed that the `APFloat::convertFromString` wasn't working the way I thought it did, and it can only produce an `snan` value if you have a string that begins `snan`. So I made a change to the IR from the original proposal to distinguish between qnan and snan using `nan`/`snan` rather than high bit of the payload.

https://github.com/llvm/llvm-project/pull/121838