[llvm] [AArch64][SME] Enable subreg liveness tracking when SME is available (PR #92142)

[Matt Arsenault via llvm-commits]

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@@ -584,6 +584,10 @@ AArch64Subtarget::getAuthenticatedLRCheckMethod() const {
   return AArch64PAuth::AuthCheckMethod::None;
 }
 
+bool AArch64Subtarget::enableSubRegLiveness() const {
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arsenm wrote:

Or we could make the -enable-subreg-liveness option actually work and not require every target to do this 

https://github.com/llvm/llvm-project/pull/92142