Rollup merge of rust-lang#74715 - oli-obk:mir_pass_diff, r=wesleywiser

Add a system for creating diffs across multiple mir optimizations.

r? @wesleywiser

Author: Manishearth

src/test/mir-opt/match-arm-scopes.rs

@@ -9,8 +9,7 @@
 //   all of the bindings for that scope.
 // * No drop flags are used.
 
-// EMIT_MIR rustc.complicated_match.SimplifyCfg-initial.after.mir
-// EMIT_MIR rustc.complicated_match.ElaborateDrops.after.mir
+// EMIT_MIR rustc.complicated_match SimplifyCfg-initial.after ElaborateDrops.after
 fn complicated_match(cond: bool, items: (bool, bool, String)) -> i32 {
     match items {
         (false, a, s) | (a, false, s) if if cond { return 3 } else { a } => 1,