SILVerifier: fix a wrong debug-scope verifier crash

Remove the "initialization loop" for LastSeenScope in verifyDebugScopeHoles.
It's not required because LastSeenScope is initialized the same way in the main loop.
And most importantly: this loop was missing the check for cleanup locations.

rdar://75374671

Author: eeckstein

lib/SIL/Verifier/SILVerifier.cpp

@@ -5465,13 +5465,6 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       return;
 
     const SILDebugScope *LastSeenScope = nullptr;
-    for (SILInstruction &SI : *BB) {
-      if (SI.isMetaInstruction())
-        continue;
-      LastSeenScope = SI.getDebugScope();
-      AlreadySeenScopes.insert(LastSeenScope);
-      break;
-    }
     for (SILInstruction &SI : *BB) {
       if (SI.isMetaInstruction())
         continue;

test/DebugInfo/guard-let-scope.swift

@@ -11,3 +11,13 @@ func f(c: AnyObject?) {
   }
   print(x)
 }
+
+// Check that we don't crash with a verifier error on this.
+protocol P {}
+
+public func testit(_ x: AnyObject) -> Bool {
+  guard let _ = x as? P else {
+    return false
+  }
+  fatalError()
+}