[x86/SLH] Add a test covering indirect forms of control flow. NFC.

This specifically covers different ways of making indirect calls and
jumps. There are some bugs in SLH that I will be fixing in subsequent
patches where the diff in the generated instructions makes the bug fix
much more clear, so just checking in a baseline of this test to start.

I'm also going to be adding direct mitigation for variant 1.2 which this
file very specifically tests in the various forms it can arise on x86.
Again, the diff to the generated instructions should make the change for
that much more clear, so having the test as a baseline seems useful.

llvm-svn: 337672