Fix mutation decomposition turning (<field-write> += <arg>) into (<field-write> = <field-write> + <arg>) instead of (<field-write> = <field-read> + arg).
Fix mutation decomposition turning (<field-write> += <arg>) into (<field-write> = <field-write> + <arg>) instead of (<field-write> = <field-read> + arg).

iskiselev/JSIL,iskiselev/JSIL,FUSEEProjectTeam/JSIL,sq/JSIL,hach-que/JSIL,sq/JSIL,hach-que/JSIL,sq/JSIL,iskiselev/JSIL,iskiselev/JSIL,iskiselev/JSIL,hach-que/JSIL,hach-que/JSIL,FUSEEProjectTeam/JSIL,FUSEEProjectTeam/JSIL,sq/JSIL,hach-que/JSIL,FUSEEProjectTeam/JSIL,FUSEEProjectTeam/JSIL,sq/JSIL