AMDGPU: Change insertion point of si_mask_branch

Insert before the skip branch if one is created.
This is a somewhat more natural placement relative
to the skip branches, and makes it possible to implement
analyzeBranch for skip blocks.

The test changes are mostly due to a quirk where
the block label is not emitted if there is a terminator
that is not also a branch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278273 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed