[tint][ir] Rename Case::start -> Case::block

Without merge blocks, the whole case is held by a single block.

Change-Id: If0fb23448abf7d36c7ae8adbe11b3f0975f5935b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/136381
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/ir/switch.cc b/src/tint/ir/switch.cc
index f8add4c..9d9e75d 100644
--- a/src/tint/ir/switch.cc
+++ b/src/tint/ir/switch.cc
@@ -28,7 +28,7 @@
 
 void Switch::ForeachBlock(const std::function<void(ir::Block*)>& cb) {
     for (auto& c : cases_) {
-        cb(c.Start());
+        cb(c.Block());
     }
 }