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/hlsl.doLoop.frag

@@ -1,9 +1,29 @@
+void f0() {
+    [unroll] do {} while (false);
+}
+
+void f1() {
+    [unroll] do {;} while (false);
+}
+
+float f2(float input) {
+    do { return (float4)input; } while (input > 2.0);
+}
+
+void f3(float input) {
+    do ++input; while (input < 10.0);
+}
+
+void f4(float input) {
+    do while (++input < 10.0); while (++input < 10.0); // nest while inside do-while
+}
+
 float4 PixelShaderFunction(float input) : COLOR0
 {
-    [unroll] do {} while (false);
-    [unroll] do {;} while (false);
-    do { return (float4)input; } while (input > 2.0);
-    do ++input; while (input < 10.0);
-    do while (++input < 10.0); while (++input < 10.0); // nest while inside do-while
+    f0();
+    f1();
+    f2(input);
+    f3(input);
+    f4(input);
     return (float4)input;
 }