Convert IRGenerator::convertIf to IfStatement::Make.

This lets the Rehydrator and Inliner benefit from static optimization
opportunities, like converting `if (false) {...}` to a Nop.

Change-Id: I70b4fceab84c50ea270dc894b0d6fe0e7e2369eb
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375777
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
8 files changed