AMDGPU/GlobalISel: Handle most function return types

AMDGPU/GlobalISel: Handle most function return types

handleAssignments gives up pretty easily on structs, and i8 values for
some reason. The other case that doesn't work is when an implicit sret
needs to be inserted if the return size exceeds the number of return
registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367082 91177308-0d34-0410-b5e6-96231b3b80d8