MandatoryPerformanceOptimizations: remove dead metatype instructions after specialization

Generic specialization drops metatype arguments and therefore exposes opportunities to remove dead metatype instructions.
Instead of removing dead metatype instructions before specialization, try to remove them after specialization.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/ModulePasses/MandatoryPerformanceOptimizations.swift

@@ -58,17 +58,15 @@ private func optimize(function: Function, _ context: FunctionPassContext) {
     switch instruction {
     case let apply as FullApplySite:
       inlineAndDevirtualize(apply: apply, context, simplifyCtxt)
-    case let mt as MetatypeInst:
-      if mt.isTriviallyDeadIgnoringDebugUses {
-        simplifyCtxt.erase(instructionIncludingDebugUses: mt)
-      }
     default:
       break
     }
   }
 
   _ = context.specializeApplies(in: function, isMandatory: true)
 
+  removeUnusedMetatypeInstructions(in: function, context)
+
   // If this is a just specialized function, try to optimize copy_addr, etc.
   if context.optimizeMemoryAccesses(in: function) {
     _ = context.eliminateDeadAllocations(in: function)
@@ -103,6 +101,15 @@ private func inlineAndDevirtualize(apply: FullApplySite, _ context: FunctionPass
   }
 }
 
+private func removeUnusedMetatypeInstructions(in function: Function, _ context: FunctionPassContext) {
+  for inst in function.instructions {
+    if let mt = inst as? MetatypeInst,
+       mt.isTriviallyDeadIgnoringDebugUses {
+      context.erase(instructionIncludingDebugUses: mt)
+    }
+  }
+}
+
 private func shouldInline(apply: FullApplySite, callee: Function) -> Bool {
   if callee.isTransparent {
     return true