ash: speed up ${v//pattern/repl} if !ASH_OPTIMIZE_FOR_SIZE

function                                             old     new   delta
subevalvar                                          1353    1447     +94
.rodata                                           104179  104184      +5
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 99/0)               Total: 99 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>