AMDGPU: Remove MFI::ABIArgOffset

We have too many mechanisms for tracking the various offsets
used for kernel arguments, so remove one. There's still a lot of
confusion with these because there are two different "implicit"
argument areas located at the beginning and end of the kernarg
segment.

Additionally, the offset was determined based on the memory
size of the split element types. This would break in a future
commit where v3i32 is decomposed into separate i32 pieces.

llvm-svn: 335830