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.functionCall.frag.out

@@ -105,8 +105,7 @@ WARNING: 0:5: varying deprecated in version 130; may be removed in future releas
               44:               Label
                                 ReturnValue 45
               41:             Label
-              47:    6(float) Undef
-                              ReturnValue 47
+                              Unreachable
                               FunctionEnd
 18(missingReturn():    6(float) Function None 15
               19:             Label