Fix #781: Handle loops that include mid-circuit measurements #787
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Due to an interaction between the revised memtoreg pass and loop unrolling, the test with a mid-circuit measurement and a break statement still wasn't being unrolled. These changes fix that issue.
Primarily, these changes allow structured operations that accept region arguments to straddle the fence and allow some values to be promoted as dominating uses (when only used) while other values to be threaded as region arguments exactly as before (when written). This change simplifies the register-semantics IR. There may be a bit of performance lossage in memtoreg as a result however as promoted values may later be discovered to be written and the changes will need to be reapplied.