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/array_element.wgsl.expected.hlsl

@@ -1,10 +1,10 @@
-SKIP: FAILED
-
-
-@group(0) @binding(0) var<storage, read_write> a : array<u32>;
-
-fn main() {
-  a[1]++;
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
 }
 
-Failed to generate: error: cannot modify value of type 'u32'
+RWByteAddressBuffer a : register(u0, space0);
+
+void main() {
+  a.Store(4u, asuint((a.Load(4u) + 1u)));
+}