ASTify array assignment
Blind fix: use _to_iv_value for numeric comparisons as well
Sanity check: don't try to use the target when it isn't there
Allow emitting comparison ops as the root of a JITted tree

This could be resolved by creating a mortal scalar in the no-target
case, but for now it's better to avoid a slow default and handle this
on a case-by case basis.