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-loop-break.vert

@@ -0,0 +1,19 @@
+#version 450
+
+layout(location =0 ) in int c;
+layout(location =0 ) out int o;
+
+void main() {
+  int i;
+  o = 1;
+  for (i=0; i < 5; i++) {
+    o = 2;
+    if (i==c) {
+      o = 3;
+      break;
+      o = 4;
+    }
+    o = 5;
+  }
+  o = 6;
+}