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.msl

@@ -1,11 +1,8 @@
-SKIP: FAILED
+#include <metal_stdlib>
 
-
-@group(0) @binding(0) var<storage, read_write> i : u32;
-
-fn tint_symbol() {
-  for(i++; (i < 10u); ) {
+using namespace metal;
+void tint_symbol(device uint* const tint_symbol_1) {
+  for(*(tint_symbol_1) = (*(tint_symbol_1) + 1u); (*(tint_symbol_1) < 10u); ) {
   }
 }
 
-Failed to generate: error: unknown statement type: tint::ast::IncrementDecrementStatement