Fix tail calling intrinsics

[WaffleLapkin]

Tail-calling intrinsics used to ICE/miscompile, this PR fixes that.

The reason I think we should explicitly support calling intrinsics, rather than banning it, is that I don't think we want to expose to users if a function is an intrinsic or not. This is especially relevant with something like std::mem::transmute which is a direct export of the intrinsic, rather than a wrapper function.

The fix has two parts:

1. For intrinsics which are lowered in mir I've added code to lower them even if they are tail called
2. For cg_ssa I added code which transforms a tail call to an intrinsic into intrinsic desugaring+ret. This feels like a hack, but I don't really see a better way. (cc @scottmcm, maybe you have an opinion?)

Fixes #144806

[rustbot]

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rust-log-analyzer]

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added 291 packages in 34s
Running eslint on rustdoc JS files
info: ES-Check: there were no ES version matching errors!  🎉
some tidy checks failed
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Build completed unsuccessfully in 0:01:51
  local time: Sat Aug  2 09:34:10 UTC 2025
  network time: Sat, 02 Aug 2025 09:34:10 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[theemathas]

The fact that transmute is an intrinsic is actually already exposed to users. It cannot be coerced to a function pointer.

fn main() {
    let _: unsafe fn(()) = std::mem::transmute::<(), ()>;
}

error[E0308]: cannot coerce intrinsics to function pointers
 --> src/main.rs:2:28
  |
2 |     let _: unsafe fn(()) = std::mem::transmute::<(), ()>;
  |            -------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot coerce intrinsics to function pointers
  |            |
  |            expected due to this
  |
  = note: expected fn pointer `unsafe fn(())`
                found fn item `unsafe fn(()) {std::intrinsics::transmute::<(), ()>}`

For more information about this error, try `rustc --explain E0308`.