[ARM] Don't use the Machine Scheduler for cortex-m at minsize

The new cortex-m schedule in rL360768 helps performance, but can increase the
amount of high-registers used. This, on average, ends up increasing the
codesize by a fair amount (because less instructions are converted from T2 to
T1). On cortex-m at -Oz, where we are quite size-paranoid, it is better to use
the existing DAG scheduler with the RegPressure scheduling preference (at least
until the issues around T2 vs T1 instructions can be improved).

I have also made sure that the Sched::RegPressure dag scheduler is always
chosen for MinSize.

The test shows one case where we increase the number of registers used.

Differential Revision: https://reviews.llvm.org/D61882

llvm-svn: 360769