AELEMFASTLEX_STORE - support negative keys, skip unnecessary check

[richardleach]

This commit:

• Adds support for negative keys, as per the original AELEMFAST_LEX
• Changes an if() check for a "useless assignment to a temporary" into an assert, since this condition should never be true when the LHS is the result of an array fetch.

Both changes address feedback from @iabyn in #20063.