[Subtarget] Merge ProcSched and ProcDesc arrays in MCSubtargetInfo into a single array.

These arrays are both keyed by CPU name and go into the same tablegenerated file. Merge them so we only need to store keys once.

This also removes a weird space saving quirk where we used the ProcDesc.size() to create to build an ArrayRef for ProcSched.

Differential Revision: https://reviews.llvm.org/D58939

llvm-svn: 355431