AMDGPU: Set SubRegIndex size and offset

I'm not sure what reasons the comment here could have
had for not setting these. Without these set, there is
an assertion hit during DWARF emission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243661 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed