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/function.wgsl.expected.hlsl

@@ -1,9 +1,9 @@
-SKIP: FAILED
-
-
-fn main() {
-  var i = 0;
-  i++;
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
 }
 
-Failed to generate: error: unknown statement type: tint::ast::IncrementDecrementStatement
+void main() {
+  int i = 0;
+  i = (i + 1);
+}