[region-isolation] Add Asserting support for {alloc,dealloc}_pack_metadata since they only appear in Lowered SIL.

I also added a note to the SIL.rst and an assert into the SILVerifier to better
document this requirement.

Author: gottesmm

docs/SIL.rst

@@ -3844,6 +3844,8 @@ Notionally, ``alloc_pack_metadata`` is a stack allocation instruction.  See the
 section above on stack discipline.  The corresponding stack deallocation
 instruction is ``dealloc_pack_metadata``.
 
+Only valid in Lowered SIL.
+
 alloc_ref
 `````````
 ::
@@ -4124,6 +4126,8 @@ instruction after its operand) must be cleaned up here.
 on Stack Discipline above.  The operand must be an ``alloc_pack_metadata``
 instruction.
 
+Only valid in Lowered SIL.
+
 dealloc_box
 ```````````
 ::

lib/SIL/Verifier/SILVerifier.cpp

@@ -6273,13 +6273,17 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
     require(apmi->getIntroducer()->mayRequirePackMetadata(),
             "Introduces instruction of kind which cannot emit on-stack pack "
             "metadata");
+    require(F.getModule().getStage() == SILStage::Lowered,
+            "Only supported in lowered SIL");
   }
 
   void checkDeallocPackMetadataInst(DeallocPackMetadataInst *dpmi) {
     auto *apmi = dpmi->getOperand()->getDefiningInstruction();
     require(apmi, "Must have instruction operand.");
     require(isa<AllocPackMetadataInst>(apmi),
             "Must have alloc_pack_metadata operand");
+    require(F.getModule().getStage() == SILStage::Lowered,
+            "Only supported in lowered SIL");
   }
 
   void checkMoveOnlyWrapperToCopyableAddrInst(

lib/SILOptimizer/Mandatory/TransferNonSendable.cpp

@@ -2409,7 +2409,6 @@ CONSTANT_TRANSLATION(YieldInst, Require)
 
 // Unhandled instructions
 CONSTANT_TRANSLATION(AllocVectorInst, Unhandled)
-CONSTANT_TRANSLATION(AllocPackMetadataInst, Unhandled)
 CONSTANT_TRANSLATION(AllocExistentialBoxInst, Unhandled)
 CONSTANT_TRANSLATION(IndexRawPointerInst, Unhandled)
 CONSTANT_TRANSLATION(UncheckedTrivialBitCastInst, Unhandled)
@@ -2464,7 +2463,6 @@ CONSTANT_TRANSLATION(RebindMemoryInst, Unhandled)
 CONSTANT_TRANSLATION(ThrowAddrInst, Unhandled)
 CONSTANT_TRANSLATION(AwaitAsyncContinuationInst, Unhandled)
 CONSTANT_TRANSLATION(DeallocPackInst, Unhandled)
-CONSTANT_TRANSLATION(DeallocPackMetadataInst, Unhandled)
 CONSTANT_TRANSLATION(DeallocStackRefInst, Unhandled)
 CONSTANT_TRANSLATION(DeallocRefInst, Unhandled)
 CONSTANT_TRANSLATION(DeallocPartialRefInst, Unhandled)
@@ -2513,6 +2511,11 @@ CONSTANT_TRANSLATION(StrongRetainUnownedInst, Asserting)
 CONSTANT_TRANSLATION(UnownedReleaseInst, Asserting)
 CONSTANT_TRANSLATION(UnownedRetainInst, Asserting)
 
+// Instructions only valid in lowered SIL. Please only add instructions here
+// after adding an assert into the SILVerifier that this property is true.
+CONSTANT_TRANSLATION(AllocPackMetadataInst, Asserting)
+CONSTANT_TRANSLATION(DeallocPackMetadataInst, Asserting)
+
 #undef CONSTANT_TRANSLATION
 
 #ifdef LOOKTHROUGH_IF_NONSENDABLE_RESULT_REQUIRE_OTHERWISE