[clang] [llvm] [X86] Fix x87 excess precision escaping into f32/f64 values (PR #215104)

[Dennis Duda via cfe-commits]

seritools wrote:

I'll get 3 of the commits out as separate PRs, as they are changes/fixes that are not specific to this PR, just needed _for_ it.

IMO the rest should stay together until some of the open questions are resolved. Maybe it makes sense to reorder it such that the opt-out becomes an opt-in, then the changes can land behind it in separate PRs, then the decision can be made to make it opt-out afterwards?

https://github.com/llvm/llvm-project/pull/215104