Function simplify_regex_expr 

pub fn simplify_regex_expr(
    left: Box<Expr>,
    op: Operator,
    right: Box<Expr>,
) -> Result<Expr>

Tries to convert a regexp expression to a LIKE or Eq/NotEq expression.

This function also validates the regex pattern. And will return error if the pattern is invalid.

Typical cases this function can simplify:

• empty regex pattern to LIKE '%'
• literal regex patterns to LIKE '%foo%'
• full anchored regex patterns (e.g. ^foo$) to = 'foo'
• partial anchored regex patterns (e.g. ^foo) to LIKE 'foo%'
• combinations (alternatives) of the above, will be concatenated with OR or AND
• EQ .* to NotNull
• NE .* means IS EMPTY

Dev note: unit tests of this function are in expr_simplifier.rs, case test_simplify_regex.