Optimizer: remove redundant `copy_block` instructions
Removes a `copy_block` if its only uses, beside ownership instructions, are callees of function calls
```
%2 = copy_block %0
%3 = begin_borrow [lexical] %2
%4 = apply %3() : $@convention(block) @noescape () -> ()
end_borrow %3
destroy_value %2
```
->
```
%4 = apply %0() : $@convention(block) @noescape () -> ()
```
rdar://118521396
