Removed remaining fields from IRGenerator

This removes all remaining fields from IRGenerator (other than fContext,
which is redundant with the one already in ThreadContext). As
IRGenerator now contains no meaningful data, the remaining work to
finish removing it is trivial, and it will go away as soon as its little
remaining code is re-homed.

Change-Id: I1815f71c14b60383c8e657411a74841386748432
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/459878
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
5 files changed