Temporarily disable the SLPVectorizer

The SLPVectorizer vectorizes loads from a packed struct of i2s.

 {<i2, i2, i2, i2>}

The problem with that is that LLVM codegens scalar accesses to such a type as
if it where a packed i8 struct.
So when we emit a vector load of <4 x i2> (i.e one byte) and extract any non
zero index element we get garbage. Disable the SLPVectorizer until it is fixed.

rdar://22673382

Swift SVN r32299

lib/IRGen/IRGen.cpp

@@ -147,7 +147,7 @@ static bool performLLVM(IRGenOptions &Opts, DiagnosticEngine &Diags,
   if (Opts.Optimize && !Opts.DisableLLVMOptzns) {
     PMBuilder.OptLevel = 3;
     PMBuilder.Inliner = llvm::createFunctionInliningPass(200);
-    PMBuilder.SLPVectorize = !Opts.DisableLLVMSLPVectorizer;
+    PMBuilder.SLPVectorize = false;
     PMBuilder.LoopVectorize = true;
   } else {
     PMBuilder.OptLevel = 0;