[ARM] Split predicates out into their own .td file

The new ARMPredicates.td is included from ARM.td, early enough that
the predicate definitions are already in scope when ARMSchedule.td is
included. This will make it possible to refer to them in
UnsupportedFeatures fields of scheduling models.

NFC: the chunk of Tablegen being moved here is copied and pasted
verbatim.

Patch by: Simon Tatham

Differential Revision: https://reviews.llvm.org/D60693

llvm-svn: 361958