image/jpeg: reduce bound checks from idct and fdct

Before -
$gotip build -gcflags="-d=ssa/check_bce/debug=1" fdct.go idct.go
./fdct.go:89:10: Found IsInBounds
./fdct.go:90:10: Found IsInBounds
./fdct.go:91:10: Found IsInBounds
./fdct.go:92:10: Found IsInBounds
./fdct.go:93:10: Found IsInBounds
./fdct.go:94:10: Found IsInBounds
./fdct.go:95:10: Found IsInBounds
./fdct.go:96:10: Found IsInBounds
./idct.go:77:9: Found IsInBounds
./idct.go:77:27: Found IsInBounds
./idct.go:77:45: Found IsInBounds
./idct.go:78:7: Found IsInBounds
./idct.go:78:25: Found IsInBounds
./idct.go:78:43: Found IsInBounds
./idct.go:78:61: Found IsInBounds
./idct.go:79:13: Found IsInBounds
./idct.go:92:13: Found IsInBounds
./idct.go:93:12: Found IsInBounds
./idct.go:94:12: Found IsInBounds
./idct.go:95:12: Found IsInBounds
./idct.go:97:12: Found IsInBounds
./idct.go:98:12: Found IsInBounds
./idct.go:99:12: Found IsInBounds

After -
$gotip build -gcflags="-d=ssa/check_bce/debug=1" fdct.go idct.go
./fdct.go:90:9: Found IsSliceInBounds
./idct.go:76:11: Found IsSliceInBounds
./idct.go:145:11: Found IsSliceInBounds

name                 old time/op    new time/op    delta
FDCT-4                 1.85µs ± 2%    1.74µs ± 1%  -5.95%  (p=0.000 n=10+10)
IDCT-4                 1.94µs ± 2%    1.89µs ± 1%  -2.67%  (p=0.000 n=10+9)
DecodeBaseline-4       1.45ms ± 2%    1.46ms ± 1%    ~     (p=0.156 n=9+10)
DecodeProgressive-4    2.21ms ± 1%    2.21ms ± 1%    ~     (p=0.796 n=10+10)
EncodeRGBA-4           24.9ms ± 1%    25.0ms ± 1%    ~     (p=0.075 n=10+10)
EncodeYCbCr-4          26.1ms ± 1%    26.2ms ± 1%    ~     (p=0.573 n=8+10)

name                 old speed      new speed      delta
DecodeBaseline-4     42.5MB/s ± 2%  42.4MB/s ± 1%    ~     (p=0.162 n=9+10)
DecodeProgressive-4  27.9MB/s ± 1%  27.9MB/s ± 1%    ~     (p=0.796 n=10+10)
EncodeRGBA-4         49.4MB/s ± 1%  49.1MB/s ± 1%    ~     (p=0.066 n=10+10)
EncodeYCbCr-4        35.3MB/s ± 1%  35.2MB/s ± 1%    ~     (p=0.586 n=8+10)

name                 old alloc/op   new alloc/op   delta
DecodeBaseline-4       63.0kB ± 0%    63.0kB ± 0%    ~     (all equal)
DecodeProgressive-4     260kB ± 0%     260kB ± 0%    ~     (all equal)
EncodeRGBA-4           4.40kB ± 0%    4.40kB ± 0%    ~     (all equal)
EncodeYCbCr-4          4.40kB ± 0%    4.40kB ± 0%    ~     (all equal)

name                 old allocs/op  new allocs/op  delta
DecodeBaseline-4         5.00 ± 0%      5.00 ± 0%    ~     (all equal)
DecodeProgressive-4      13.0 ± 0%      13.0 ± 0%    ~     (all equal)
EncodeRGBA-4             4.00 ± 0%      4.00 ± 0%    ~     (all equal)
EncodeYCbCr-4            4.00 ± 0%      4.00 ± 0%    ~     (all equal)

Updates #24499

Change-Id: I6828d077b851817503a7c1a08235763f81bdadf9
Reviewed-on: https://go-review.googlesource.com/c/go/+/167417


Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>