Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

compiler/rustc_hir/src/intravisit.rs

@@ -1289,6 +1289,7 @@ pub fn walk_inline_asm<'v, V: Visitor<'v>>(
             InlineAsmOperand::SymStatic { path, .. } => {
                 try_visit!(visitor.visit_qpath(path, id, *op_sp));
             }
+            InlineAsmOperand::Label { block } => try_visit!(visitor.visit_block(block)),
         }
     }
     V::Result::output()