Revert change to lEmitVaryingSelect().

Using vector select versus a store and masked load for varying vector
selects seems to give worse code.  This may be related to
http://llvm.org/bugs/show_bug.cgi?id=16941.

expr.cpp

@@ -3123,7 +3123,10 @@ static llvm::Value *
 lEmitVaryingSelect(FunctionEmitContext *ctx, llvm::Value *test,
                    llvm::Value *expr1, llvm::Value *expr2,
                    const Type *type) {
-#if !defined(LLVM_3_1)
+#if 0 // !defined(LLVM_3_1)
+    // Though it should be equivalent, this seems to cause non-trivial
+    // performance regressions versus the below.  This may be related to
+    // http://llvm.org/bugs/show_bug.cgi?id=16941.
     if (test->getType() != LLVMTypes::Int1VectorType)
         test = ctx->TruncInst(test, LLVMTypes::Int1VectorType);
     return ctx->SelectInst(test, expr1, expr2, "select");