Use const repl optimisation with s///e where possible

In those cases where s///e contains a single variable or a sequence
that is folded to a const op, we can do away with substcont.

PMf_EVAL means that there was an /e.  But we don’t actually need to
check that; instead we can just examine the op tree, which we have to
do anyway.

The op tree that s//$x/e and s//"constant"/e compile down to have a
null (a do-block) containing a scope op (block with a single state-
ment, as opposed to op_leave which represents multiple statements)
containing a null followed by the constant or variable.

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