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.dead-after-return.vert.out

@@ -0,0 +1,29 @@
+spv.dead-after-return.vert
+// Module Version 10000
+// Generated by (magic number): 80007
+// Id's are bound by 14
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Vertex 4  "main" 8 13
+                              Source GLSL 450
+                              Name 4  "main"
+                              Name 8  "o"
+                              Name 13  "c"
+                              Decorate 8(o) Location 0
+                              Decorate 13(c) Location 0
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeInt 32 1
+               7:             TypePointer Output 6(int)
+            8(o):      7(ptr) Variable Output
+               9:      6(int) Constant 1
+              11:      6(int) Constant 3
+              12:             TypePointer Input 6(int)
+           13(c):     12(ptr) Variable Input
+         4(main):           2 Function None 3
+               5:             Label
+                              Store 8(o) 9
+                              Return
+                              FunctionEnd