"_" discards array-ref assignments

[quasilyte]

If expression has visible result, assignment to "" should not hide it.
This means it is invalid to remove "" assignments completely.

Currently, there is function call as an exception, it should be extended with
array/slice/string lookups, pointer dereferences and other expressions that may panic.

Example in Go std code:

case i <= rune2Max:
		_ = p[1] // eliminate bounds checks