[AVR] Fix incorrect source regclass of LDWRdPtr

Summary:
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.

Reviewers: dylanmckay

Reviewed By: dylanmckay

Subscribers: dylanmckay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62300

llvm-svn: 362351