Honor lowp/mediump/highp precision qualifiers in IRGenerator.

This CL does not update the DSLParser to honor these precision
qualifiers; that will be done in a followup.

Change-Id: Ib629bc99c0e6c7afb550a381d4e3b6ccc26aa64e
Bug: skia:12248
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436337
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
11 files changed