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Now that we can reliably handle forward references to `BlockAddress` (r214563), change the mechanics to simplify predicting use-list order. Previously, we created dummy `GlobalVariable`s to represent block addresses. After every function was materialized, we'd go through any forward references to its blocks and RAUW them with a proper `BlockAddress` constant. This causes some (potentially a lot of) unnecessary use-list churn, since any constant expression that it's a part of will need to be rematerialized as well. Instead, pre-construct a `BasicBlock` immediately -- without attaching it to its (empty) `Function` -- and use that to construct a `BlockAddress`. This constant will not have to be regenerated. When the function body is parsed, hook this pre-constructed basic block up in the right place using `BasicBlock::insertInto()`. Both before and after this change, the IR is temporarily in an invalid state that gets resolved when `materializeForwardReferencedFunctions()` gets called. This is a prep commit that's part of PR5680, but the only functionality change is the reduction of churn in the constant pool. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214570 91177308-0d34-0410-b5e6-96231b3b80d8
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