[x86] Teach the EFLAGS copy lowering to handle much more complex control

flow patterns including forks, merges, and even cyles.

This tries to cover a reasonably comprehensive set of patterns that
still don't require PHIs or PHI placement. The coverage was inspired by
the amazing variety of patterns produced when copy EFLAGS and restoring
it to implement Speculative Load Hardening. Without this patch, we
simply cannot make such complex and invasive changes to x86 instruction
sequences due to EFLAGS.

I've added "just" one test, but this test covers many different
complexities and corner cases of this approach. It is actually more
comprehensive, as far as I can tell, than anything that I have
encountered in the wild on SLH.

Because the test is so complex, I've tried to give somewhat thorough
comments and an ASCII-art diagram of the control flows to make it a bit
easier to read and maintain long-term.

Differential Revision: https://reviews.llvm.org/D49220

llvm-svn: 336985