[hl] optimize closure null check

[yuxiaomao]

The goal was to remove some null check when we know that the closure is valid, when working on #13043:

.14    @5 staticclosure 2, fun$34
.14    @6 jnotnull 2,2                   ; <- not necessary
.14    @7 null 3
.14    @8 jalways 1
.14    @9 instanceclosure 3, fun$32(2)
.14    @A setfield 0[0],3

and becomes

.14    @5 staticclosure 2, fun$34
.14    @6 instanceclosure 3, fun$32(2)
.14    @7 setfield 0[0],3

I'll probably test it a little bit more and write some hlcode tests later