Improve how the MIR dialect/phase index is reported.

The only visible change is to the filenames produce by `-Zdump-mir`.
E.g. before and after:
```
h.main.003-000.analysis-post-cleanup.after.mir
h.main.2-2-000.analysis-post-cleanup.after.mir
```
It also fixes a FIXME comment.

compiler/rustc_middle/src/mir/mod.rs

@@ -100,20 +100,13 @@ impl<'tcx> HasLocalDecls<'tcx> for Body<'tcx> {
 }
 
 impl MirPhase {
-    /// Gets the index of the current MirPhase within the set of all `MirPhase`s.
-    ///
-    /// FIXME(JakobDegen): Return a `(usize, usize)` instead.
-    pub fn phase_index(&self) -> usize {
-        const BUILT_PHASE_COUNT: usize = 1;
-        const ANALYSIS_PHASE_COUNT: usize = 2;
-        match self {
-            MirPhase::Built => 1,
-            MirPhase::Analysis(analysis_phase) => {
-                1 + BUILT_PHASE_COUNT + (*analysis_phase as usize)
-            }
-            MirPhase::Runtime(runtime_phase) => {
-                1 + BUILT_PHASE_COUNT + ANALYSIS_PHASE_COUNT + (*runtime_phase as usize)
-            }
+    /// Gets the (dialect, phase) index of the current `MirPhase`. Both numbers
+    /// are 1-indexed.
+    pub fn index(&self) -> (usize, usize) {
+        match *self {
+            MirPhase::Built => (1, 1),
+            MirPhase::Analysis(analysis_phase) => (2, 1 + analysis_phase as usize),
+            MirPhase::Runtime(runtime_phase) => (3, 1 + runtime_phase as usize),
         }
     }
 

compiler/rustc_middle/src/mir/pretty.rs

@@ -231,7 +231,8 @@ fn dump_path<'tcx>(
     let pass_num = if tcx.sess.opts.unstable_opts.dump_mir_exclude_pass_number {
         String::new()
     } else if pass_num {
-        format!(".{:03}-{:03}", body.phase.phase_index(), body.pass_count)
+        let (dialect_index, phase_index) = body.phase.index();
+        format!(".{}-{}-{:03}", dialect_index, phase_index, body.pass_count)
     } else {
         ".-------".to_string()
     };

compiler/rustc_middle/src/mir/syntax.rs

@@ -38,7 +38,7 @@ use crate::ty::{self, GenericArgsRef, List, Region, Ty, UserTypeAnnotationIndex}
 /// those restrictions. I.e. to convert MIR from one phase to the next might require
 /// removing/replacing certain MIR constructs.
 ///
-/// When adding dialects or phases, remember to update [`MirPhase::phase_index`].
+/// When adding dialects or phases, remember to update [`MirPhase::index`].
 #[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
 #[derive(HashStable)]
 pub enum MirPhase {