[sil] Ensure that all SILValues have a parent function by making it so that SILUndef is uniqued at the function instead of module level.

For years, optimizer engineers have been hitting a common bug caused by passes
assuming all SILValues have a parent function only to be surprised by SILUndef.
Generally we see SILUndef not that often so we see this come up later in
testing. This patch eliminates that problem by making SILUndef uniqued at the
function level instead of the module level. This ensures that it makes sense for
SILUndef to have a parent function, eliminating this possibility since we can
define an API to get its parent function.

rdar://123484595

lib/SILOptimizer/LoopTransforms/LoopRotate.cpp

@@ -151,7 +151,8 @@ static void updateSSAForUseOfValue(
   assert(Res->getType() == MappedValue->getType() && "The types must match");
 
   insertedPhis.clear();
-  updater.initialize(Res->getType(), Res->getOwnershipKind());
+  updater.initialize(MappedValue->getFunction(), Res->getType(),
+                     Res->getOwnershipKind());
   updater.addAvailableValue(Header, Res);
   updater.addAvailableValue(EntryCheckBlock, MappedValue);