This change implements if-else blocks as well as a few simple binary
operations (equality, less-than etc).  It builds on prior work to
generate the function in separate blocks and introduces a "conditionals"
stack to support nesting of if-else blocks, switches and loops.

Bug: angleproject:4889
Change-Id: If7694000487811837ed5946753568b41d67199f0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2929660
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>