Instead of using an `if` node that has a list of `else` statements,
make each `if` statement have a single optional `else` statement,
which may itself be an `if` statement (or just a block statement).
This better matches the WGSL grammar (now that we have removed
`elseif`), and simplifies various pieces of code that handle these
statements.
Change-Id: Ie4272f1422224490ac598a03aa8b4dd00ba03010
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/87940
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: James Price <jrprice@google.com>
26ebe5ec36