Add a RegisterClassInfo class that lazily caches information about

register classes.

It provides information for each register class that cannot be
determined statically, like:

- The number of allocatable registers in a class after filtering out the
  reserved and invalid registers.

- The preferred allocation order with registers that overlap callee-saved
  registers last.

- The last callee-saved register that overlaps a given physical register.

This information usually doesn't change between functions, so it is
reused for compiling multiple functions when possible.  The many
possible combinations of reserved and callee saves registers makes it
unfeasible to compute this information statically in TableGen.

Use RegisterClassInfo to count available registers in various heuristics
in SimpleRegisterCoalescing, making the pass run 4% faster.

llvm-svn: 132450