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.glsl

@@ -1,10 +1,13 @@
-SKIP: FAILED
+#version 310 es
 
-
-@group(0) @binding(0) var<storage, read_write> a : array<u32>;
-
-fn main() {
-  a[1]++;
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
+void unused_entry_point() {
+  return;
+}
+layout(binding = 0, std430) buffer a_block_1 {
+  uint inner[];
+} a;
+void tint_symbol() {
+  a.inner[1] = (a.inner[1] + 1u);
 }
 
-Failed to generate: error: unknown statement type: tint::ast::IncrementDecrementStatement