DeadStoreElimination: fix a corner case of storing a trivial case of a non-trivial enum

Storing a trivial enum case in a non-trivial enum must be treated like a non-trivial init or assign, e.g.
```
  %1 = enum $Optional<String>, #Optional.none!enumelt
  store %1 to [trivial] %0  // <- cannot delete this store!
  store %2 to [assign] %0
```

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift

@@ -167,8 +167,15 @@ private extension StoreInst {
 
   var hasValidOwnershipForDeadStoreElimination: Bool {
     switch storeOwnership {
-    case .unqualified, .trivial:
+    case .unqualified:
       return true
+    case .trivial:
+      // Storing a trivial enum case in a non-trivial enum must be treated like a non-trivial
+      // init or assign, e.g.
+      //   %1 = enum $Optional<String>, #Optional.none!enumelt
+      //   store %1 to [trivial] %0  // <- cannot delete this store!
+      //   store %2 to [assign] %0
+      return source.type.isTrivial(in: parentFunction)
     case .initialize, .assign:
       // In OSSA, non-trivial values cannot be dead-store eliminated because that could shrink
       // the lifetime of the original stored value (because it's not kept in memory anymore).

test/SILOptimizer/dead_store_elim.sil

@@ -1615,6 +1615,17 @@ bb0(%0 : $Int, %1 : @owned $IntAndFoo, %2 : $*IntAndFoo):
   return %r : $()
 }
 
+// We don't support this, yet.
+// Just check that we don't crash.
+sil [ossa] @test_trivial_store_of_non_trivial_enum : $@convention(thin) (@owned Optional<String>) -> @out Optional<String> {
+bb0(%0 : $*Optional<String>, %1 : @owned $Optional<String>):
+  %2 = enum $Optional<String>, #Optional.none!enumelt
+  store %2 to [trivial] %0
+  store %1 to [assign] %0
+  %r = tuple ()
+  return %r
+}
+
 // CHECK-LABEL: sil @test_bind_memory  :
 // CHECK:         store %0
 // CHECK:       end sil function 'test_bind_memory'