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/Spv.FromFile.cpp

@@ -63,6 +63,7 @@ std::string FileNameAsCustomTestSuffixIoMap(
 }
 
 using CompileVulkanToSpirvTest = GlslangTest<::testing::TestWithParam<std::string>>;
+using CompileVulkanToSpirvDeadCodeElimTest = GlslangTest<::testing::TestWithParam<std::string>>;
 using CompileVulkanToDebugSpirvTest = GlslangTest<::testing::TestWithParam<std::string>>;
 using CompileVulkan1_1ToSpirvTest = GlslangTest<::testing::TestWithParam<std::string>>;
 using CompileToSpirv14Test = GlslangTest<::testing::TestWithParam<std::string>>;
@@ -85,6 +86,13 @@ TEST_P(CompileVulkanToSpirvTest, FromFile)
                             Target::Spv);
 }
 
+TEST_P(CompileVulkanToSpirvDeadCodeElimTest, FromFile)
+{
+    loadFileCompileAndCheck(GlobalTestSettings.testRoot, GetParam(),
+                            Source::GLSL, Semantics::Vulkan, glslang::EShTargetVulkan_1_0, glslang::EShTargetSpv_1_0,
+                            Target::Spv);
+}
+
 // Compiling GLSL to SPIR-V with debug info under Vulkan semantics. Expected
 // to successfully generate SPIR-V.
 TEST_P(CompileVulkanToDebugSpirvTest, FromFile)
@@ -417,6 +425,23 @@ INSTANTIATE_TEST_CASE_P(
     FileNameAsCustomTestSuffix
 );
 
+// Cases with deliberately unreachable code.
+// By default the compiler will aggressively eliminate
+// unreachable merges and continues.
+INSTANTIATE_TEST_CASE_P(
+    GlslWithDeadCode, CompileVulkanToSpirvDeadCodeElimTest,
+    ::testing::ValuesIn(std::vector<std::string>({
+        "spv.dead-after-continue.vert",
+        "spv.dead-after-discard.frag",
+        "spv.dead-after-return.vert",
+        "spv.dead-after-loop-break.vert",
+        "spv.dead-after-switch-break.vert",
+        "spv.dead-complex-continue-after-return.vert",
+        "spv.dead-complex-merge-after-return.vert",
+    })),
+    FileNameAsCustomTestSuffix
+);
+
 // clang-format off
 INSTANTIATE_TEST_CASE_P(
     Glsl, CompileVulkanToDebugSpirvTest,