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.

gtests/TestFixture.h

@@ -113,7 +113,7 @@ public:
           forceVersionProfile(false),
           isForwardCompatible(false) {
         // Perform validation by default.
-        validatorOptions.validate = true;
+        spirvOptions.validate = true;
     }
 
     // Tries to load the contents from the file at the given |path|. On success,
@@ -693,14 +693,14 @@ public:
                                     expectedOutputFname, result.spirvWarningsErrors);
     }
 
-    glslang::SpvOptions& options() { return validatorOptions; }
+    glslang::SpvOptions& options() { return spirvOptions; }
 
 private:
     const int defaultVersion;
     const EProfile defaultProfile;
     const bool forceVersionProfile;
     const bool isForwardCompatible;
-    glslang::SpvOptions validatorOptions;
+    glslang::SpvOptions spirvOptions;
 };
 
 }  // namespace glslangtest