[AVR] Fix incorrect register state for LDRdPtr

Summary:
LDRdPtr expanded from LDWRdPtr shouldn't define its second operand(SrcReg).
The second operand is its source register.
Add -verify-machineinstrs into command line of testcases can trigger this error.

Reviewers: dylanmckay

Reviewed By: dylanmckay

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75437