ARM: Remove override of shouldRewriteCopySrc

All of the overrides of shouldRewriteCopySrc appear to be hacks
for bugs in the base implementation, so I'm trying to delete
all of the overrides. I was expecting this to find an example
issue like the x86 version, but no tests change with this.

Author: arsenm

llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp

@@ -961,17 +961,3 @@ bool ARMBaseRegisterInfo::shouldCoalesce(MachineInstr *MI,
   }
   return false;
 }
-
-bool ARMBaseRegisterInfo::shouldRewriteCopySrc(const TargetRegisterClass *DefRC,
-                                               unsigned DefSubReg,
-                                               const TargetRegisterClass *SrcRC,
-                                               unsigned SrcSubReg) const {
-  // We can't extract an SPR from an arbitary DPR (as opposed to a DPR_VFP2).
-  if (DefRC == &ARM::SPRRegClass && DefSubReg == 0 &&
-      SrcRC == &ARM::DPRRegClass &&
-      (SrcSubReg == ARM::ssub_0 || SrcSubReg == ARM::ssub_1))
-    return false;
-
-  return TargetRegisterInfo::shouldRewriteCopySrc(DefRC, DefSubReg,
-                                                  SrcRC, SrcSubReg);
-}

llvm/lib/Target/ARM/ARMBaseRegisterInfo.h

@@ -159,11 +159,6 @@ class ARMBaseRegisterInfo : public ARMGenRegisterInfo {
                       const TargetRegisterClass *NewRC,
                       LiveIntervals &LIS) const override;
 
-  bool shouldRewriteCopySrc(const TargetRegisterClass *DefRC,
-                            unsigned DefSubReg,
-                            const TargetRegisterClass *SrcRC,
-                            unsigned SrcSubReg) const override;
-
   int getSEHRegNum(unsigned i) const { return getEncodingValue(i); }
 };