flow follows a while body's statements and applies each increment it
meets as a statement. An increment inside an expression, such as an if
condition's comma expression `if ((i++, i > 0))`, lowers to a statement
nested in the expression; whileBound accepted it as an increment, but
flow never applied it, so a read after it kept the pre-increment bound
and lost its bounds check. whileBound now gives no bound when any
expression in the body holds a write of the counter.
Chris Tate
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