After we've modified the prolog to save volatile registers, generate the compact

After we've modified the prolog to save volatile registers, generate the compact
unwind encoding for that function. This simply crawls through the prolog looking
for machine instrs marked as "frame setup". It can calculate from these what the
compact unwind should look like.

This is currently disabled because of needed linker support. But initial tests
look good.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135922 91177308-0d34-0410-b5e6-96231b3b80d8