transform/InlinePtrLets: Fix ICE for lets in for-loops

For loop initializers and continuing statements do not have a BlockStatement as their parent.
Handle removal of these statements with a new Transform::RemoveStatement() helper

Fixed: tint:990
Change-Id: I24e7b18dcf71d3ef0a4d3ee68b9f68518e0eb5e8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58063
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>

test/bug/tint/990.wgsl

@@ -0,0 +1,4 @@
+fn f() {
+    var i : i32;
+    for (let p = &i;;) {}
+}

test/bug/tint/990.wgsl.expected.hlsl

@@ -0,0 +1,12 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  int i = 0;
+  {
+    for(; ; ) {
+    }
+  }
+}

test/bug/tint/990.wgsl.expected.msl

@@ -0,0 +1,9 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void f() {
+  int i = 0;
+  for(; ; ) {
+  }
+}
+

test/bug/tint/990.wgsl.expected.spvasm

@@ -0,0 +1,35 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 15
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
+               OpExecutionMode %unused_entry_point LocalSize 1 1 1
+               OpName %unused_entry_point "unused_entry_point"
+               OpName %f "f"
+               OpName %i "i"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+%_ptr_Function_int = OpTypePointer Function %int
+         %10 = OpConstantNull %int
+%unused_entry_point = OpFunction %void None %1
+          %4 = OpLabel
+               OpReturn
+               OpFunctionEnd
+          %f = OpFunction %void None %1
+          %6 = OpLabel
+          %i = OpVariable %_ptr_Function_int Function %10
+               OpBranch %11
+         %11 = OpLabel
+               OpLoopMerge %12 %13 None
+               OpBranch %14
+         %14 = OpLabel
+               OpBranch %13
+         %13 = OpLabel
+               OpBranch %11
+         %12 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/bug/tint/990.wgsl.expected.wgsl

@@ -0,0 +1,5 @@
+fn f() {
+  var i : i32;
+  for(let p = &(i); ; ;) {
+  }
+}