AMDGPU/SI: Only look at live out SGPR defs

When trying to fix SGPR live ranges, skip defs that are
killed in the same block as the def. I don't think
we need to worry about these cases as long as the
live ranges of the SGPRs in dominating blocks are
correct.

This reduces the number of elements the second
loop over the function needs to look at, and makes
it generally easier to understand. The second loop
also only considers if the live range is live
in to a block, which logically means it
must have been live out from another.

llvm-svn: 245150