[SILOptimizer] Epilogue matcher can find partial post dom argument releases.

Added getPartiallyPostDomReleaseSet to
ConsumedArgToEpilogueReleaseMatcher.  Given an argument, the new method
returns the array of releases of the argument if there is an array
thereof and if the releases therein do not jointly post-dominate the
argument.

Excerpted from @GottesM's swiftlang#16756.

Author: nate-chandler

include/swift/SILOptimizer/Analysis/ARCAnalysis.h

@@ -332,6 +332,17 @@ class ConsumedArgToEpilogueReleaseMatcher {
     return completeList.getValue();
   }
 
+  Optional<ArrayRef<SILInstruction *>>
+  getPartiallyPostDomReleaseSet(SILArgument *arg) const {
+    auto iter = ArgInstMap.find(arg);
+    if (iter == ArgInstMap.end())
+      return None;
+    auto partialList = iter->second.getPartiallyPostDomReleases();
+    if (!partialList)
+      return None;
+    return partialList;
+  }
+
   ArrayRef<SILInstruction *> getReleasesForArgument(SILValue value) const {
     auto *arg = dyn_cast<SILArgument>(value);
     if (!arg)