[sksl][spirv] Prevent unreachable blocks in loops

- Do not output statements that follow a continue/break statement
  inside loop block.
- Avoid writing an unreachable block when writing do-while statements.

Bug: skia:13484
Change-Id: Ifeac58b6dbd58e2fab7ca7b0233676f2b4bc0260
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/555657
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed