Review/fix the coalesce and sort operations on PreorderAST nodes to account for the possibility of overflow

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1125

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The coalesce and sort operations that are performed on the nodes of a PreorderAST implicitly re-associate the operands of + and * operators. If an expression like P + (E1 + E2) where the subexpression P has a checked pointer type and the subexpressions E1 and E2 have integral types, is re-associated as (P + E1) + E2, this is fine as this re-association does not introduce any new overflow scenarios that are not caught by runtime checks on checked pointer arithmetic. Whereas, if an expression like (P + E1) + E2 is re-associated as P + (E1 + E2), then there is a possibility that the subexpression (E1 + E2) may overflow and this overflow is not detected at runtime. Such a re-association should be performed by the compiler only if it can verify at compile-time that the subexpression (E1 + E2) will not overflow.