[NFC] SILVerifier: Doc + cleanup to store_borrow.

Author: nate-chandler

lib/SIL/Verifier/SILVerifier.cpp

@@ -2450,23 +2450,22 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
   }
 
   void checkStoreBorrowInst(StoreBorrowInst *SI) {
+    // A store_borrow must be to an alloc_stack.  That alloc_stack can only be
+    // used by store_borrows (and dealloc_stacks).
     require(SI->getSrc()->getType().isObject(),
             "Can't store from an address source");
     require(!fnConv.useLoweredAddresses()
                 || SI->getSrc()->getType().isLoadable(*SI->getFunction()),
             "Can't store a non loadable type");
     require(SI->getDest()->getType().isAddress(),
             "Must store to an address dest");
+    // Note: This is the current implementation and the design is not final.
     require(isa<AllocStackInst>(SI->getDest()),
-            "store_borrow destination should be alloc_stack");
+            "store_borrow destination can only be an alloc_stack");
     requireSameType(SI->getDest()->getType().getObjectType(),
                     SI->getSrc()->getType(),
                     "Store operand type and dest type mismatch");
 
-    // Note: This is the current implementation and the design is not final.
-    require(isa<AllocStackInst>(SI->getDest()),
-            "store_borrow destination can only be an alloc_stack");
-
     SSAPrunedLiveness scopedAddressLiveness;
     ScopedAddressValue scopedAddress(SI);
     // FIXME: Reenable @test_load_borrow_store_borrow_nested in

test/SIL/store_borrow_verify_errors.sil

@@ -196,7 +196,7 @@ bb1:
 }
 
 // CHECK: Begin Error in function test_store_borrow_dest
-// CHECK: SIL verification failed: store_borrow destination should be alloc_stack: isa<AllocStackInst>(SI->getDest())
+// CHECK: SIL verification failed: store_borrow destination can only be an alloc_stack: isa<AllocStackInst>(SI->getDest())
 // CHECK: Verifying instruction:
 // CHECK:    %0 = argument of bb0 : $Klass                  // user: %3
 // CHECK:      %2 = struct_element_addr %1 : $*NonTrivialStruct, #NonTrivialStruct.val // user: %3