IR: better array len instruction
IR: better array len instruction

reference the "len" field directly instead of looking it up by
name since we already know exactly where the field metadata is

andrewrk/zig,Dimenus/zig,Dimenus/zig,raulgrell/zig,raulgrell/zig,Dimenus/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig