SPIR-V: Aggressively prune unreachable merge, continue target

More aggressively prune unreachable code as follows.
When no control flow edges reach a merge block or continue target:
- delete their contents so that:
  - a merge block becomes OpLabel, then OpUnreachable
  - a continue target becomes OpLabel, then an OpBranch back to the
    loop header
- any basic block which is dominated by such a merge block or continue
  target is removed as well.
- decorations targeting the removed instructions are removed.

Enables the SPIR-V builder post-processing step the GLSLANG_WEB case.

Test/spv.dead-after-switch-break.vert

@@ -0,0 +1,15 @@
+#version 450
+
+layout(location =0 ) in int c;
+layout(location =0 ) out int o;
+
+void main() {
+  int i;
+  switch(c) {
+    case 0: o=1;
+      break;
+      o=2;
+   default: break;
+  }
+  o = 3;
+}