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/hir.rs

@@ -2650,6 +2650,9 @@ pub enum InlineAsmOperand<'hir> {
         path: QPath<'hir>,
         def_id: DefId,
     },
+    Label {
+        block: &'hir Block<'hir>,
+    },
 }
 
 impl<'hir> InlineAsmOperand<'hir> {
@@ -2659,7 +2662,10 @@ impl<'hir> InlineAsmOperand<'hir> {
             | Self::Out { reg, .. }
             | Self::InOut { reg, .. }
             | Self::SplitInOut { reg, .. } => Some(reg),
-            Self::Const { .. } | Self::SymFn { .. } | Self::SymStatic { .. } => None,
+            Self::Const { .. }
+            | Self::SymFn { .. }
+            | Self::SymStatic { .. }
+            | Self::Label { .. } => None,
         }
     }
 
@@ -2680,6 +2686,12 @@ pub struct InlineAsm<'hir> {
     pub line_spans: &'hir [Span],
 }
 
+impl InlineAsm<'_> {
+    pub fn contains_label(&self) -> bool {
+        self.operands.iter().any(|x| matches!(x.0, InlineAsmOperand::Label { .. }))
+    }
+}
+
 /// Represents a parameter in a function header.
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub struct Param<'hir> {