Add missing addressing mode
Do not use the stack when registers are available

BenchmarkVerticalScaler2          5304         4786   -9.77%
BenchmarkVerticalScaler4          7935         7480   -5.73%
BenchmarkVerticalScaler6         10495         9965   -5.05%
BenchmarkVerticalScaler8         11690        11315   -3.21%
BenchmarkVerticalScaler10        14385        13845   -3.75%
BenchmarkVerticalScaler12        16500        15950   -3.33%
BenchmarkVerticalScalerN         19371        18801   -2.94%
Load memory as soon as possible

benchmark                    old ns/op    new ns/op    delta
BenchmarkVerticalScaler2          4840         4840   +0.00%
BenchmarkVerticalScaler4          7550         7094   -6.04%
BenchmarkVerticalScaler6         10055         9365   -6.86%
BenchmarkVerticalScaler8         11420        11015   -3.55%
BenchmarkVerticalScaler10        13955        13450   -3.62%
BenchmarkVerticalScaler12        16100        15520   -3.60%
BenchmarkVerticalScalerN         19011        18671   -1.79%
Simplify horizontal 4-taps

It's apparently a litte faster, but not by much

benchmark                     old ns/op    new ns/op    delta
BenchmarkHorizontalScaler4        10270         9955   -3.07%
Factorize some common code
Simplify horizontal 8-taps

It's also a tiny bit faster due to memory loads done sooner

benchmark                      old ns/op    new ns/op    delta
BenchmarkHorizontalScaler6         16320        14925   -8.55%
BenchmarkHorizontalScaler8         16310        14955   -8.31%
Factorize more code