AMDGPU: Insert skip branches over return blocks

AMDGPU: Insert skip branches over return blocks

SIInsertSkips really doesn't understand the control flow, and makes
very stupid assumptions about the block layout. This was able to get
away with not skipping return blocks, since usually after
structurization there is only one placed at the end of the
function. Tail duplication can break this assumption.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362754 91177308-0d34-0410-b5e6-96231b3b80d8