[IR] LangRef: state explicitly that floats generally behave according to IEEE-754 (#102140)

Fixes https://github.com/llvm/llvm-project/issues/60942: IEEE semantics
is likely what many frontends want (it definitely is what Rust wants),
and it is what LLVM passes already assume when they use APFloat to
propagate float operations.

This does not reflect what happens on x87, but what happens there is
just plain unsound (https://github.com/llvm/llvm-project/issues/89885,
https://github.com/llvm/llvm-project/issues/44218); there is no coherent
specification that will describe this behavior correctly -- the backend
in combination with standard LLVM passes is just fundamentally buggy in
a hard-to-fix-way.

There's also the questions around flushing subnormals to zero, but [this
discussion](https://discourse.llvm.org/t/questions-about-llvm-canonicalize/79378)
seems to indicate a general stance of: this is specific non-standard
hardware behavior, and generally needs LLVM to be told that basic float
ops do not return the standard result. Just naively running
LLVM-compiled code on hardware configured to flush subnormals will lead
to #89885-like issues.

AFAIK this is also what Alive2 implements (@nunoplopes please correct me
if I am wrong).

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