[PATCH] Do not transform PHI to select if doing so would be unsafe

[David Majnemer]

PR16069 is an interesting case where an incoming value to a PHI is a trap
value while also being a 'ConstantExpr'.

We do not consider this case when performing the 'HoistThenElseCodeToIf'
optimization.

The attached patch will make our modifications more conservative if we
detect that we cannot transform the PHI to a select.

-- 
David Majnemer
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