Allow address-phis with -enforce-exclusivity.

As long as begin/end access scopes are respected, enforce-exclusivity does not
actually care about address phis.

Eventually, we will disable address phis completely, but that requires adding a
"address legalization" transform to various CFG passes, such as jump threading
and loop rotate.

Meanwhile, we will continue to suppress these CFG transformations when access
markers are involved. This means that optimizations will work differently, and
some will be suppressed in the presence of access markers. That means that
optimizing the access markers themselves will be a prerequisite to enabling them
by default, even if the overhead of the checks is tolerable.

Author: atrick

lib/SIL/SILVerifier.cpp

@@ -329,20 +329,15 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       delete Dominance;
   }
 
-  // Address-type block args must be prohibited whenever access markers are
-  // present. Access markers are always present in raw SIL to diagnose exclusive
-  // memory access. In canonical SIL, access markers are only present with
-  // EnforceExclusivityDynamic.
-  //
   // FIXME: For sanity, address-type block args should be prohibited at all SIL
   // stages. However, the optimizer currently breaks the invariant in three
   // places:
   // 1. Normal Simplify CFG during conditional branch simplification
   //    (sneaky jump threading).
   // 2. Simplify CFG via Jump Threading.
   // 3. Loop Rotation.
-  // Once EnforceExclusivityDynamic is performant enough to be enabled by
-  // default at -O, address-type blocks args can be prohibited unconditionally.
+  //
+  //
   bool prohibitAddressBlockArgs() {
     // If this function was deserialized from canonical SIL, this invariant may
     // already have been violated regardless of this module's SIL stage or
@@ -352,12 +347,7 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       return false;
 
     SILModule &M = F.getModule();
-    if (M.getStage() == SILStage::Raw)
-      return true;
-
-    // If dynamic enforcement is enabled, markers are present at -O so we
-    // prohibit address block args.
-    return M.getOptions().EnforceExclusivityDynamic;
+    return M.getStage() == SILStage::Raw;
   }
 
   void visitSILArgument(SILArgument *arg) {