AArch64: Enable subregister liveness tracking.

[MatzeB]

Summary

Splitting this into an own commit to get some visibility.

After doing some extensive benchmarking wiht llvm test-suite, I see no correctness or compiletime changes. I see some very slight benchmark runtime improvements (which may or may not be noise). The only noticeable regression is in a benchmark were literally 6 lines of assembly changed for the better, so that must be cache issues.

[MatzeB]

Abandoning this: There are no measurable performance benefits, so this is not worth the compiletime. We don't need this for testing the subreg infrastructure as AMDGPU (and an out-of-tree target) already have it enabled.

lib/Target/AArch64/AArch64Subtarget.h

public:
   const AArch64RegisterInfo *getRegisterInfo() const override {
     return &getInstrInfo()->getRegisterInfo();
   }
   const Triple &getTargetTriple() const { return TargetTriple; }
   bool enableMachineScheduler() const override { return true; }
   bool enablePostMachineScheduler() const override {
     return isCortexA53() || isCortexA57();
   }
+  bool enableSubRegLiveness() const override { return true; }
 
   bool hasZeroCycleRegMove() const { return HasZeroCycleRegMove; }
 
   bool hasZeroCycleZeroing() const { return HasZeroCycleZeroing; }
 
   bool hasFPARMv8() const { return HasFPARMv8; }
   bool hasNEON() const { return HasNEON; }
   bool hasCrypto() const { return HasCrypto; }