Storing if conditions to temp vars to fix issue #420 #544
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As pointed out in issue #420, Carbon optimizes away empty if-statements in the generated Boogie code.
This can be problematic if the if-condition contains a triggering term needed to prove something, which is then also optimized away.
This PR fixes that by always storing if-conditions to an auxiliary variable and using the variable in the if-condition on the Boogie level, so that the existing optimization can no longer lead to important triggering terms disappearing.