Fix deparsing of newline and tab with (?x)

B::Deparse was taking special care to preserve whitespace for /x pat-
terns, and avoid changing newlines to \n, etc.  But it was not tak-
ing (?x) into account.  Parsing regular expressions to apply the
right escapes to different parts of the regexp is too compli-
cated, so this commit changes it to treat all regular expres-
sions the way it was treating /x expressions.  This causes
‘cd t; ./perl TEST -deparse run/switchM.t’ to pass.
deparse-skips.txt: Note skip reason for switchd-78586.t
deparse-skips.txt: run/switches.t passes
Fix deparsing of (1,2,3) in void context

This was broken in v5.21.5-289-g12cea2f.  We were trying to check the
kid op of an ex-const, but an ex-const has no children.