Fixes for bugs around unreachable code

Remove the ICE check for expression behaviors always being either `{Next}` or `{Next, Discard}`. Unreachable code may be result in something else.

Add the RemoveUnreachableStatements transform to the SPIR-V writer sanitizer transform list. The writer cannot correctly handle unreachable statements.

Bug: tint:1369
Bug: chromium:1285622
Change-Id: I9fa54c6d2096b1ee633dd551b628c7dd3ba64fb5
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/76300
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/bug/tint/1369.wgsl

@@ -0,0 +1,10 @@
+fn call_discard() -> bool {
+  discard;
+  return true;
+}
+
+[[stage(fragment)]]
+fn f() {
+    var v = call_discard();
+    var also_unreachable : bool;
+}

test/bug/tint/1369.wgsl.expected.hlsl

@@ -0,0 +1,22 @@
+bug/tint/1369.wgsl:3:3 warning: code is unreachable
+  return true;
+  ^^^^^^
+
+bug/tint/1369.wgsl:9:9 warning: code is unreachable
+    var also_unreachable : bool;
+        ^^^^^^^^^^^^^^^^
+
+bool call_discard() {
+  if (true) {
+    discard;
+    return true;
+  }
+  bool unused;
+  return unused;
+}
+
+void f() {
+  bool v = call_discard();
+  bool also_unreachable = false;
+  return;
+}

test/bug/tint/1369.wgsl.expected.msl

@@ -0,0 +1,22 @@
+bug/tint/1369.wgsl:3:3 warning: code is unreachable
+  return true;
+  ^^^^^^
+
+bug/tint/1369.wgsl:9:9 warning: code is unreachable
+    var also_unreachable : bool;
+        ^^^^^^^^^^^^^^^^
+
+#include <metal_stdlib>
+
+using namespace metal;
+bool call_discard() {
+  discard_fragment();
+  return true;
+}
+
+fragment void f() {
+  bool v = call_discard();
+  bool also_unreachable = false;
+  return;
+}
+

test/bug/tint/1369.wgsl.expected.spvasm

@@ -0,0 +1,37 @@
+bug/tint/1369.wgsl:3:3 warning: code is unreachable
+  return true;
+  ^^^^^^
+
+bug/tint/1369.wgsl:9:9 warning: code is unreachable
+    var also_unreachable : bool;
+        ^^^^^^^^^^^^^^^^
+
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 13
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %f "f"
+               OpExecutionMode %f OriginUpperLeft
+               OpName %call_discard "call_discard"
+               OpName %f "f"
+               OpName %v "v"
+       %bool = OpTypeBool
+          %1 = OpTypeFunction %bool
+       %void = OpTypeVoid
+          %5 = OpTypeFunction %void
+%_ptr_Function_bool = OpTypePointer Function %bool
+         %12 = OpConstantNull %bool
+%call_discard = OpFunction %bool None %1
+          %4 = OpLabel
+               OpKill
+               OpFunctionEnd
+          %f = OpFunction %void None %5
+          %8 = OpLabel
+          %v = OpVariable %_ptr_Function_bool Function %12
+          %9 = OpFunctionCall %bool %call_discard
+               OpStore %v %9
+               OpReturn
+               OpFunctionEnd

test/bug/tint/1369.wgsl.expected.wgsl

@@ -0,0 +1,18 @@
+bug/tint/1369.wgsl:3:3 warning: code is unreachable
+  return true;
+  ^^^^^^
+
+bug/tint/1369.wgsl:9:9 warning: code is unreachable
+    var also_unreachable : bool;
+        ^^^^^^^^^^^^^^^^
+
+fn call_discard() -> bool {
+  discard;
+  return true;
+}
+
+[[stage(fragment)]]
+fn f() {
+  var v = call_discard();
+  var also_unreachable : bool;
+}