S_check_for_bool_cxt(): avoid OPpTARGET_MY + bool

S_check_for_bool_cxt(): avoid OPpTARGET_MY + bool

Where an op allows the OA_TARGLEX optimisation (which eliminates the
assign op in '$lex = foo' and just sets the targ directly - which may be a
padtmp or $lex - the op doesn't care), disallow setting a boolean context
flag on that op. Because if the op does something like

    if (OPpTRUEBOOL)
        SETs(cond ? &PL_sv_yes : &PL_sv_no)

skipping assigning to TARG, then $lex won't get set.