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/spv.controlFlowAttributes.frag

@@ -4,11 +4,18 @@
 
 bool cond;
 
+void f0() {
+        [[loop]]                   for (;;) { }
+}
+
+void f1() {
+        [[dont_unroll]]            while(true) {  }
+}
+
 void main()
 {
         [[unroll]]                 for (int i = 0; i < 8; ++i) { }
-        [[loop]]                   for (;;) { }
-        [[dont_unroll]]            while(true) {  }
+	f0();
         [[dependency_infinite]]    do {  } while(true);
         [[dependency_length(1+3)]] for (int i = 0; i < 8; ++i) { }
         [[flatten]]                if (cond) { } else { }