tint: Refactor if-else statement representation

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>

docs/tint/compound_statements.md

@@ -24,13 +24,11 @@ sem::IfStatement {
     sem::BlockStatement {
         statement_a
     }
-    sem::ElseStatement {
+    sem::IfStatement {
         condition_b
         sem::BlockStatement {
             statement_b
         }
-    }
-    sem::ElseStatement {
         sem::BlockStatement {
             statement_c
         }