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-discard.frag

@@ -0,0 +1,10 @@
+#version 450
+
+layout(location =0 ) in float c;
+layout(location =0 ) out int o;
+
+void main() {
+  o = 1;
+  discard;
+  o = 3;
+}