Change ARM / Thumb2 addc / adde and subc / sube modeling to use physical

register dependency (rather than glue them together). This is general
goodness as it gives scheduler more freedom. However it is motivated by
a nasty bug in isel.

When a i64 sub is expanded to subc + sube.
  libcall #1
     \
      \        subc 
       \       /  \
        \     /    \
         \   /    libcall #2
          sube

If the libcalls are not serialized (i.e. both have chains which are dag
entry), legalizer can serialize them in arbitrary orders. If it's
unlucky, it can force libcall #2 before libcall #1 in the above case.

  subc
   |
  libcall #2
   |
  libcall #1
   |
  sube

However since subc and sube are "glued" together, this ends up being a
cycle when the scheduler combine subc and sube as a single scheduling
unit.

The right solution is to fix LegalizeType too chains the libcalls together.
However, LegalizeType is not processing nodes in order so that's harder than
it should be. For now, the move to physical register dependency will do.

rdar://10019576

llvm-svn: 138791