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/spv.forwardFun.frag.out

@@ -99,8 +99,7 @@ spv.forwardFun.frag
               45:               Label
                                 ReturnValue 46
               42:             Label
-              48:    8(float) Undef
-                              ReturnValue 48
+                              Unreachable
                               FunctionEnd
     16(foo(vf4;):    8(float) Function None 14
          15(bar):     13(ptr) FunctionParameter