[ARM] Make -mcpu=generic schedule for an in-order core (Cortex-A8).

The benchmarking summarized in
http://lists.llvm.org/pipermail/llvm-dev/2017-May/113525.html showed
this is beneficial for a wide range of cores.

As is to be expected, quite a few small adaptations are needed to the
regressions tests, as the difference in scheduling results in:
- Quite a few small instruction schedule differences.
- A few changes in register allocation decisions caused by different
 instruction schedules.
- A few changes in IfConversion decisions, due to a difference in
 instruction schedule and/or the estimated cost of a branch mispredict.

llvm-svn: 306514