[AVR] Fix incorrect source regclass of LDWRdPtr
LDWRdPtr would be expanded to ld+ldd. ldd only accepts the pointer register is Y or Z.
So the register class of pointer of LDWRdPtr should be PTRDISPREGS instead of PTRREGS.
Reviewed By: dylanmckay
Subscribers: dylanmckay, llvm-commits
Differential Revision: https://reviews.llvm.org/D62300