The SPIR-V transformer performs multiple transformations in one pass.
This is the first of a series of refactoring changes that detangle these
transformations and encapsulate them, while allowing the transformation
to remain single-pass.

Bug: angleproject:4888
Change-Id: I327f693d99496a0b89028abe627119e4a2b07c42
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2702651Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>