fix Issue 6178 - Struct inside the AA are not init correctly

If the AA value setting invokes opAsign method, it would need additional runtime const to distinguish construction and assignment.
If struct does not invoke implicit constructor call, AA indexing for opAssign call should always invoke read access.
Allow AA value setting through 'alias this'

If the key does not yet exist in AA, the assignment always throws RangeError.
fix Issue 10595 - Using alias this and a hash generates wrong code
fix Issue 10970 - Segfault in a simple test compiled without -g.

Fixes `CondExp::toElem` to avoid "Internal error: backend\cgcs.c 351"
Merge pull request #2539 from 9rnsr/fix6178

Issue 6178 - Struct inside the AA are not init correctly