Identify common sub-expressions in printExprWithStats

[mbasmanova]

Enhance printExprWithStats to identify common-sub expressions.

For example, c0 + c1 is a common sub-expression in
"(c0 + c1) % 5", " (c0 + c1) % 3" expression set. It is evaluated only once and
there is a single Expr object that represents it. That object appears in the
expression tree twice. printExprWithStats does not show the runtime stats for
second instance of that expression and instead annotates it with [CSE #2],
where CSE stands for common sub-expression and 2 refers to the first instance
of the expression.

mod [cpu time: 50.49us, rows: 1024] -> BIGINT [#1]
   cast(plus as BIGINT) [cpu time: 68.15us, rows: 1024] -> BIGINT [#2]
      plus [cpu time: 51.84us, rows: 1024] -> INTEGER [#3]
         c0 [cpu time: 0ns, rows: 0] -> INTEGER [#4]
         c1 [cpu time: 0ns, rows: 0] -> INTEGER [#5]
   5:BIGINT [cpu time: 0ns, rows: 0] -> BIGINT [#6]

mod [cpu time: 49.29us, rows: 1024] -> BIGINT [#7]
   cast((plus(c0, c1)) as BIGINT) -> BIGINT [CSE #2]
   3:BIGINT [cpu time: 0ns, rows: 0] -> BIGINT [#8]

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[aditi-pandit]

LGTM. Thanks Masha !