[AArch64][SME] Enable subreg liveness tracking when SME is available

The SME dot instructions in these tests operate on contiguous register
tuples which use one subregister from each of the loads. When using the
strided register form for all loads, enabling subreg liveness tracking
will allow us to recognise that there is no overlap between the register
tuples used by each of the dot instructions.

This is the first in a series of patches to improve the allocation of
strided and contiguous registers for SME.

Author: kmclaughlin-arm

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

@@ -572,6 +572,10 @@ AArch64Subtarget::getAuthenticatedLRCheckMethod() const {
   return AArch64PAuth::AuthCheckMethod::None;
 }
 
+bool AArch64Subtarget::enableSubRegLiveness() const {
+  return hasSME() && isStreaming();
+}
+
 bool AArch64Subtarget::enableMachinePipeliner() const {
   return getSchedModel().hasInstrSchedModel();
 }

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -149,6 +149,7 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
   const Triple &getTargetTriple() const { return TargetTriple; }
   bool enableMachineScheduler() const override { return true; }
   bool enablePostRAScheduler() const override { return usePostRAScheduler(); }
+  bool enableSubRegLiveness() const override;
 
   bool enableMachinePipeliner() const override;
   bool useDFAforSMS() const override { return false; }