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/remap.similar_1a.everything.frag.out

@@ -88,7 +88,7 @@ remap.similar_1a.everything.frag
            22102:    649(ptr) Variable Function
            24151:     12(int) Load 4408
            13868:     9(bool) SGreaterThan 24151 2577
-                              SelectionMerge 22309 None
+                              SelectionMerge 14966 None
                               BranchConditional 13868 9492 17416
             9492:               Label
            15624:     12(int)   Load 4408
@@ -109,7 +109,6 @@ remap.similar_1a.everything.frag
            10505:     12(int)   IAdd 11462 21176
            14626:   13(float)   ConvertSToF 10505
                                 ReturnValue 14626
-           22309:             Label
-            6429:   13(float) Undef
-                              ReturnValue 6429
+           14966:             Label
+                              Unreachable
                               FunctionEnd