Rollup merge of #83916 - Amanieu:asm_anonconst, r=petrochenkov

Use AnonConst for asm! constants

This replaces the old system which used explicit promotion. See #83169 for more background.

The syntax for `const` operands is still the same as before: `const <expr>`.

Fixes #83169

Because the implementation is heavily based on inline consts, we suffer from the same issues:
- We lose the ability to use expressions derived from generics. See the deleted tests in `src/test/ui/asm/const.rs`.
- We are hitting the same ICEs as inline consts, for example #78174. It is unlikely that we will be able to stabilize this before inline consts are stabilized.

compiler/rustc_mir/src/dataflow/move_paths/builder.rs

@@ -425,7 +425,7 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
                 for op in operands {
                     match *op {
                         InlineAsmOperand::In { reg: _, ref value }
-                        | InlineAsmOperand::Const { ref value } => {
+                         => {
                             self.gather_operand(value);
                         }
                         InlineAsmOperand::Out { reg: _, late: _, place, .. } => {
@@ -441,7 +441,8 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
                                 self.gather_init(out_place.as_ref(), InitKind::Deep);
                             }
                         }
-                        InlineAsmOperand::SymFn { value: _ }
+                        InlineAsmOperand::Const { value: _ }
+                        | InlineAsmOperand::SymFn { value: _ }
                         | InlineAsmOperand::SymStatic { def_id: _ } => {}
                     }
                 }