Fix assertion from empty switch over uninhabited enum

lib/SILGen/SILGenPattern.cpp

@@ -983,6 +983,11 @@ chooseNecessaryColumn(const ClauseMatrix &matrix, unsigned firstRow) {
 /// Recursively emit a decision tree from the given pattern matrix.
 void PatternMatchEmission::emitDispatch(ClauseMatrix &clauses, ArgArray args,
                                         const FailureHandler &outerFailure) {
+  if (clauses.rows() == 0) {
+    SGF.B.createUnreachable(SILLocation(PatternMatchStmt));
+    return;
+  }
+
   unsigned firstRow = 0;
   while (true) {
     // If there are no rows remaining, then we fail.

test/Sema/exhaustive_switch.swift

@@ -360,6 +360,10 @@ enum MyNever {}
 func ~= (_ : MyNever, _ : MyNever) -> Bool { return true }
 func myFatalError() -> MyNever { fatalError() }
 
+@_frozen public enum UninhabitedT4<A> {
+  case x(A)
+}
+
 func checkUninhabited() {
   // Scrutinees of uninhabited type may match any number and kind of patterns
   // that Sema is willing to accept at will.  After all, it's quite a feat to
@@ -379,6 +383,10 @@ func checkUninhabited() {
     case myFatalError(): break
     }
   }
+
+  func test4(x: UninhabitedT4<Never>) {
+    switch x {} // No diagnostic.
+  }
 }
 
 enum Runcible {