DeadFunctionElimination: keep global variables alive which are referenced from another global

Fixes a compiler crash in case a private global is only reference from another global variable, for example:

```
private var g1 = 27
let g2 = UnsafePointer(&g1)
```

rdar://117189962

Author: eeckstein

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -220,6 +220,8 @@ class DeadFunctionAndGlobalElimination {
     for (const SILInstruction &initInst : *global) {
       if (auto *fRef = dyn_cast<FunctionRefInst>(&initInst))
         ensureAlive(fRef->getReferencedFunction());
+      if (auto *gRef = dyn_cast<GlobalAddrInst>(&initInst))
+        ensureAlive(gRef->getReferencedGlobal());
     }
   }
 

test/SILOptimizer/dead_function_elimination.swift

@@ -1,6 +1,11 @@
 // RUN: %target-swift-frontend %s -O -emit-sil | %FileCheck %s
 // RUN: %target-swift-frontend %s -O -emit-sil -enable-testing | %FileCheck -check-prefix=CHECK-TESTING %s
 
+// Check if a private global is kept alive if it's only reference from another global variable.
+
+private var g1 = 27
+let g2 = UnsafePointer(&g1)
+
 // Check if cycles are removed.
 
 @inline(never)
@@ -184,6 +189,8 @@ public func keepPtrAlive() {
   GFStr.aliveFuncPtr()
 }
 
+// CHECK-LABEL: sil_global private @$s25dead_function_elimination2g1{{.*}}
+
 // CHECK-NOT: sil {{.*}}inCycleA
 // CHECK-NOT: sil {{.*}}inCycleB
 // CHECK-NOT: sil {{.*}}DeadMethod

test/SILOptimizer/remove_unused_func.sil

@@ -25,6 +25,30 @@ sil_global @globalFunctionPointer : $@callee_guaranteed () -> () = {
   %initval = thin_to_thick_function %0 : $@convention(thin) () -> () to $@callee_guaranteed () -> ()
 }
 
+// CHECK-LABEL: sil_global private @self_referencing_private_global
+sil_global private @self_referencing_private_global : $Builtin.RawPointer = {
+  %0 = global_addr @self_referencing_private_global : $*Builtin.RawPointer
+  %initval = address_to_pointer %0 : $*Builtin.RawPointer to $Builtin.RawPointer
+}
+
+// CHECK-LABEL: sil_global private @referencing_other_private_global
+sil_global private @referencing_other_private_global : $Builtin.RawPointer = {
+  %0 = global_addr @self_referencing_private_global : $*Builtin.RawPointer
+  %initval = address_to_pointer %0 : $*Builtin.RawPointer to $Builtin.RawPointer
+}
+
+// CHECK-LABEL: sil_global @referencing_private_globals
+sil_global @referencing_private_globals : $Builtin.RawPointer = {
+  %0 = global_addr @referencing_other_private_global : $*Builtin.RawPointer
+  %initval = address_to_pointer %0 : $*Builtin.RawPointer to $Builtin.RawPointer
+}
+
+// KEEP-NOT: @dead_self_referencing_private_global
+sil_global private @dead_self_referencing_private_global : $Builtin.RawPointer = {
+  %0 = global_addr @dead_self_referencing_private_global : $*Builtin.RawPointer
+  %initval = address_to_pointer %0 : $*Builtin.RawPointer to $Builtin.RawPointer
+}
+
 // CHECK-LABEL: sil private @alivePrivateFunc
 sil private @alivePrivateFunc : $@convention(thin) () -> () {
 bb0: