[Statepoints] Separate out logic for statepoint directives; NFC

This splits out the logic that maps the `"statepoint-id"` attribute into
the actual statepoint ID, and the `"statepoint-num-patch-bytes"`
attribute into the number of patchable bytes the statpeoint is lowered
into.  The new home of this logic is in IR/Statepoint.cpp, and this
refactoring will support similar functionality when lowering calls with
deopt operand bundles in the future.

llvm-svn: 263685