Add indexTrieRoot to the SILModule to share across Analyses.

...and avoid reallocation.

This is immediately necessary for LICM, in addition to its current
uses. I suspect this could be used by many passes that work with
addresses. RLE/DSE should absolutely migrate to it.

lib/SIL/IR/SILModule.cpp

@@ -94,7 +94,8 @@ class SILModule::SerializationCallback final
 
 SILModule::SILModule(llvm::PointerUnion<FileUnit *, ModuleDecl *> context,
                      Lowering::TypeConverter &TC, const SILOptions &Options)
-    : Stage(SILStage::Raw), Options(Options), serialized(false),
+    : Stage(SILStage::Raw), indexTrieRoot(new IndexTrieNode()),
+      Options(Options), serialized(false),
       regDeserializationNotificationHandlerForNonTransparentFuncOME(false),
       regDeserializationNotificationHandlerForAllFuncOME(false),
       SerializeSILAction(), Types(TC) {