[llvm] [AArch64][SME] Spill p-regs as z-regs when streaming hazards are possible (PR #123752)

[Benjamin Maxwell via llvm-commits]

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@@ -86,6 +86,11 @@ static cl::alias AArch64StreamingStackHazardSize(
     cl::desc("alias for -aarch64-streaming-hazard-size"),
     cl::aliasopt(AArch64StreamingHazardSize));
 
+static cl::opt<bool> EnableZPRPredicateSpills(
+    "aarch64-enable-zpr-predicate-spills", cl::init(false), cl::Hidden,
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MacDue wrote:

I don't think we should right now (possibly with more analysis). If there are streaming memory hazards and predicate spills, this can help. However, I've seen a case were there's only predicate spills (outside the prologue/epilogue), where enabling this will simply replace the predicate reload with a more expensive reload. 

https://github.com/llvm/llvm-project/pull/123752