Disable expr rewrites and codegen for VALUES() statements

The analysis of statements with big VALUES clauses like INSERT INTO <tbl> VALUES is slow due to expression rewrites like constant folding. The performance of such statements has regressed since the introduction of expr rewrites and constant folding in IMPALA-1788.

We should skip expr rewrites for VALUES altogether since it mostly provides no benefit but can have a large overhead due to evaluation of expressions in the backend (constant folding). These expressions are ultimately evaluated and materialized in the backend anyway, so there's no point in folding them during analysis.

Similarly, there is no point in doing codegen for these exprs in the backend union node.

Workaround

SET ENABLE_EXPR_REWRITES=FALSE;
SET DISABLE_CODEGEN=TRUE;