There were two problems here.
1. The inline predicates `isInitialized` and `isValueInitialized` on
`ArrayAggregateLiteral` caused their callers to materialize every
`int` that was a valid index into the array. This was slow on huge
value-initialized arrays.
2. The `isInitialized` predicate was used in the `TInstructionTag` IPA
type, creating a numbered tuple for each integer in it. This seemed
to be entirely unnecessary since the `TranslatedElement`s using those
tags were already indexed appropriately.
