Optimize `Combinational._execute` X-driving

[mkorbel1]

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Motivation

The _execute function in Combinational currently drives all outputs to x, then executes the conditionals. This is less efficient than it could be, since Conditional.execute returns a list of signals driven.

Desired solution

Only drive signals to x if they are not driven by any other sub-Conditional as determined by the Conditional.execute function.