simd_fmin/fmax: make semantics and name consistent with scalar intrinsics

[RalfJung]

This is the SIMD version of #153343: change the documented semantics of the SIMD float min/max intrinsics to that of the scalar intrinsics, and also make the name consistent.

In terms of providers of this API:

• Miri, GCC, and cranelift already implement the new semantics, so no changes are needed.
• LLVM is adjusted to use minimumnum instead of minnum, thus giving us the new semantics.

In terms of consumers of this API:

• Portable SIMD almost certainly wants to match the scalar behavior, so this is strictly a bugfix here.
• Stdarch mostly stopped using the intrinsic, except on nvptx, where arguably the new semantics are closer to what we actually want than the old semantics (nvptx: Incorrect use of LLVM intrinsics for f16x2_min/max(_nan) stdarch#2056).

Q: Should there be an f in the intrinsic name to indicate that it is for floats? E.g., simd_fminimum_number_nsz?

Also see #153395.

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the platform-builtins intrinsics. Make sure the
LLVM backend as well as portable-simd gets adapted for the changes.

cc @antoyo, @GuillaumeGomez, @bjorn3, @calebzulawski, @programmerjake

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

Portable SIMD is developed in its own repository. If possible, consider making this change to rust-lang/portable-simd instead.

cc @calebzulawski, @programmerjake

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

[rustbot]

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[RalfJung]

Looks like the previous PR changed the behavior of both the scalar and SIMD intrinsics in cranelift oops. Well, with this PR that actually matches what we document then. ;)

[programmerjake]

the llvm and portable-simd parts lgtm.

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