improve cold_path()

[x17jiri]

#120370 added a new instrinsic cold_path() and used it to fix likely and unlikely

However, in order to limit scope, the information about cold code paths is only used in 2-target switch instructions. This is sufficient for likely and unlikely, but limits usefulness of cold_path for idiomatic rust. For example, code like this:

if let Some(x) = y { ... }

may generate 3-target switch:

switch y.discriminator:
0 => true branch
1 = > false branch
_ => unreachable

and therefore marking a branch as cold will have no effect.

This PR improves cold_path() to work with arbitrary switch instructions.

Note that for 2-target switches, we can use llvm.expect, but for multiple targets we need to manually emit branch weights. I checked Clang and it also emits weights in this situation. The Clang's weight calculation is more complex that this PR, which I believe is mainly because switch in C/C++ can have multiple cases going to the same target.

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[x17jiri]

Just found that this also works:

	let new_buf = Global.allocate(layout).map_err(|_| {
		cold_path();
		Error::new_alloc_failed("Cannot allocate memory.")
	})?;

As long as the closure is inlined, branch weights will be emited.

[estebank]

I'm not the best person to review this, sorry

r? compiler

[bors]

☔ The latest upstream changes (presumably #130060) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

r? compiler

[lcnr]

maybe r? @nikic 😅