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/baseResults/hlsl.structbuffer.rw.frag.out

@@ -303,6 +303,5 @@ gl_FragCoord origin is upper left
               66:    9(fvec4)   CompositeConstruct 65 65 65 65
                                 ReturnValue 66
               45:             Label
-              68:    9(fvec4) Undef
-                              ReturnValue 68
+                              Unreachable
                               FunctionEnd