Optimize load data assignment expressions

[gengliqi]

Enhancement

According to the above CPU flame graph, most of the time spends on rewriteAstExpr.
The root cause is that the ASTs of assignment expressions in load data are rewritten for each row, which may make the load data super slow.

#43492 has already fixed this issue, but only the TiDB clusters whose versions are greater than or equal to v7.2 contain this pr. In addition, there are some PR for load data code refactoring in v7.1.

So a tiny fix for this issue is needed for the previous TiDB version.