[llvm] [RegScavenger] Simplify state tracking for backwards scavenging (PR #71202)

[Jay Foad via llvm-commits]

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@@ -299,7 +299,7 @@ bool AArch64SpeculationHardening::instrumentControlFlow(
     if (I == MBB.begin())
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jayfoad wrote:

The `I == MBB.begin()` is not needed. We could clean this up further by just calling `RS.backward(I)` unconditionally. I left it in just in case there was some performance reason for having a special case for start-of-block.

https://github.com/llvm/llvm-project/pull/71202