Switch the NEON register class from QPR to DPair.

The already allocatable DPair superclass contains odd-even D register
pair in addition to the even-odd pairs in the QPR register class. There
is no reason to constrain the set of D register pairs that can be used
for NEON values. Any NEON instructions that require a Q register will
automatically constrain the register class to QPR.

The allocation order for DPair begins with the QPR registers, so
register allocation is unlikely to change much.

llvm-svn: 199186

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -156,7 +156,7 @@ void ARMTargetLowering::addDRTypeForNEON(MVT VT) {
 }
 
 void ARMTargetLowering::addQRTypeForNEON(MVT VT) {
-  addRegisterClass(VT, &ARM::QPRRegClass);
+  addRegisterClass(VT, &ARM::DPairRegClass);
   addTypeForNEON(VT, MVT::v2f64, MVT::v4i32);
 }