cmd/compile: search for remaining WB ops from end to beginning
The writebarrier pass processes WB ops from beginning to end,
replacing them by other values.
But it also checks whether there are more ops to process
by walking from beginning to end.
This is quadratic, so walk from end to beginning instead.
This speeds up compiling the code in issue 13554:
name old time/op new time/op delta
Pkg 11.9s ± 2% 8.3s ± 3% -29.88% (p=0.000 n=18+17)