MandatoryPerformanceOptimizations: don't crash with empty global-init-once functions

Instead ignore empty global-init-once functions

#73487
rdar://130041582

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/ModulePasses/MandatoryPerformanceOptimizations.swift

@@ -355,6 +355,9 @@ private extension Value {
 private extension Function {
   /// Analyzes the global initializer function and returns global it initializes (from `alloc_global` instruction).
   func getInitializedGlobal() -> GlobalVariable? {
+    if !isDefinition {
+      return nil
+    }
     for inst in self.entryBlock.instructions {
       switch inst {
       case let agi as AllocGlobalInst:

test/SILOptimizer/mandatory_performance_optimizations.sil

@@ -167,6 +167,11 @@ bb0:
   return %6 : $()
 }
 
+// Check that we don't crash on global init-once declarations.
+
+// CHECK-LABEL: sil [global_init_once_fn] [no_locks] @external_global_init_once : $@convention(c) () -> ()
+sil [global_init_once_fn] [no_locks] @external_global_init_once : $@convention(c) () -> ()
+
 sil @yield_int_value : $@convention(thin) @yield_once () -> (@yields Int32) {
 bb0:
   %0 = integer_literal $Builtin.Int32, 10