[llvm] [DeadArgElim] fix verifier failure when changing musttail's function signature (PR #127366)

[via llvm-commits]

u4f3 wrote:


Previously:

```
surveyFunction:	
	... a lot of other staff
	if Fn musttail call anotherFn:
		if we can't analyze anotherFn/somehow not internal:
			freeze Fn, we can't change Fn's signature
		end
	end

	if Fn is musttail called by others or has musttail calls:
		mark all arguments to be live, so we can't change them
	end
end surveyFunction

propagateVirtMustcallLiveness:
	all frozen fns' musttail caller should be frozen too.
end propagateVirtMustcallLiveness
```


So we will be able to somehow change the return type in `removeDeadStuffFromFunction` . We use insert and extract to change the return type. It will only be legal if we change the caller’s and callee’s return type to the same return type at the same time, otherwise we will fail the verifier. So #107569 crashes.

As for #126817,  the return type of `C` is changed to void since nobody use the return value of the internal function…So the verifier fails again. 

```llvm
define i64 @A() {
entry:
  %v2660 = musttail call i64 @B()
  ret i64 %v2660
}

define internal i64 @B() {
entry:
  ret i64 0
}

define internal i64 @C() {
entry:
  %v30543 = musttail call i64 @B()
  ret i64 %v30543
} 
```

So according to llvm langref: [LLVM Language Reference Manual — LLVM 21.0.0git documentation](https://llvm.org/docs/LangRef.html#id334). If the cc is not tailcc of swiftcc, we need to ensure they have exactly the same signature. Otherwise we only need to ensure the share the same return type here in DeadArgElim. So I change the logic to:

```
surveyFunction:	
	... a lot of other staff
	if Fn musttail call anotherFn:
		freeze the whole Fn or just the return type according to cc, and if we freeze the whole Fn, return.
	end

	if Fn is musttail called by others:
		freeze the whole Fn or just the return type according to cc, and if we freeze the whole Fn, return.
	end
end surveyFunction

propagateVirtMustcallLiveness:
	No need for this since if the cc is not tailcc of swifttailcc, we already mark both the caller and callee frozen.
end propagateVirtMustcallLiveness
```

https://github.com/llvm/llvm-project/pull/127366