Fix PR18572 - llc crash during GenericScheduler::initPolicy().

Generalized the heuristic that looks at the (very rough) size of the
register file before enabling regpressure tracking.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199766 91177308-0d34-0410-b5e6-96231b3b80d8