Add a really faster pre-RA scheduler (-pre-RA-sched=linearize). It doesn't use

any scheduling heuristics nor does it build up any scheduling data structure
that other heuristics use. It essentially linearize by doing a DFA walk but
it does handle glues correctly.

IMPORTANT: it probably can't handle all the physical register dependencies so
it's not suitable for x86. It also doesn't deal with dbg_value nodes right now
so it's definitely is still WIP.

rdar://12474515

llvm-svn: 166122