[llvm] [Coroutines] Only rematerialize when operands are available after the suspend (PR #209195)

[Weibo He via llvm-commits]

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@@ -316,9 +405,13 @@ void coro::doRematerializations(
   // See if there are materializable instructions across suspend points
   // We record these as the starting point to also identify materializable
   // defs of uses in these operations
+  SmallDenseMap<Value *, bool> AvailMemo;
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NewSigma wrote:

The place of these comments is somewhat confusing. Please
1. sink them
2. add a comment like 'Stores whether the value is available after suspend to `AvailMemo`.

https://github.com/llvm/llvm-project/pull/209195