Disallow cond_br with identical destinations.

It avoids generation of llvm phi nodes with identical predecessors and differing values.
This change replaces my previous fix of this problem in r23580, where I handled it in IRGen.

There were some discussions about it with the conclusion that it's better to just disallow such cond_br instructions in SIL.
It makes the life easier for some SIL optimizations which can't deal with cond_br with identical destinations.

The original radar is <rdar://problem/18568272> Swift compiler fails with "PHI node has multiple entries for the same basic block with different incoming values!"



Swift SVN r23861

lib/SIL/Verifier.cpp

@@ -2118,6 +2118,8 @@ public:
 
     require(CBI->getTrueArgs().size() == CBI->getTrueBB()->bbarg_size(),
             "true branch has wrong number of arguments for dest bb");
+    require(CBI->getTrueBB() != CBI->getFalseBB(),
+            "identical destinations");
     require(std::equal(CBI->getTrueArgs().begin(), CBI->getTrueArgs().end(),
                       CBI->getTrueBB()->bbarg_begin(),
                       [](SILValue branchArg, SILArgument *bbArg) {