AMDGPU: Adjust the chain for loads writing to the HI part of a register.

Summary:
  For these loads that write to the HI part of a register, we should chain them to the op that writes to the LO part
of the register to maintain the appropriate order.

Reviewers:
  rampitec, arsenm

Differential Revision:
  https://reviews.llvm.org/D56454

llvm-svn: 351379