Add support for increment/decrement statements

Refactor the ExpandCompoundAssignment transform to handle these
statements, which delivers support for all of the non-WGSL backends.

Fixed: tint:1488
Change-Id: I96cdc31851c61f6d92d296447d0b0637907d5fe5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86004
Reviewed-by: Ben Clayton <bclayton@google.com>

test/tint/statements/increment/for_loop_initializer.wgsl.expected.hlsl

@@ -1,11 +1,13 @@
-SKIP: FAILED
-
-
-@group(0) @binding(0) var<storage, read_write> i : u32;
-
-fn main() {
-  for(i++; (i < 10u); ) {
-  }
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
 }
 
-Failed to generate: error: cannot modify value of type 'u32'
+RWByteAddressBuffer i : register(u0, space0);
+
+void main() {
+  {
+    [loop] for(i.Store(0u, asuint((i.Load(0u) + 1u))); (i.Load(0u) < 10u); ) {
+    }
+  }
+}