nir: Remove fake edges in the CF handling code

nir: Remove fake edges in the CF handling code

When NIR was first introduced, Connor added this fake-edge hack to work
around issues related to unreachable blocks.  Thanks to GLSL IR's jump
lowering code, the only unreachable code you can have is a block after an
infinite loop.  With SPIR-V, we didn't have the jump lowering code so we
could also end up with the "if (...) { break; } else { continue; }" case
which generates an unreachable block after the if.  Because of this, most
of NIR had to be fixed up for handling unreachable blocks.  The only
remaining case of not handling unreachable blocks was specifically the
block-after-infinite-loop case in dead_cf which was fixed by the previous
commit.  We can now delete the fake edge hack.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>