arm: vp9itxfm: Do a simpler half/quarter idct16/idct32 when possible

This work is sponsored by, and copyright, Google.

This avoids loading and calculating coefficients that we know will
be zero, and avoids filling the temp buffer with zeros in places
where we know the second pass won't read.

This gives a pretty substantial speedup for the smaller subpartitions.

The code size increases from 12388 bytes to 19784 bytes.

The idct16/32_end macros are moved above the individual functions; the
instructions themselves are unchanged, but since new functions are added
at the same place where the code is moved from, the diff looks rather
messy.

Before:                              Cortex A7       A8       A9      A53
vp9_inv_dct_dct_16x16_sub1_add_neon:     273.0    189.5    212.0    235.8
vp9_inv_dct_dct_16x16_sub2_add_neon:    2102.1   1521.7   1736.2   1265.8
vp9_inv_dct_dct_16x16_sub4_add_neon:    2104.5   1533.0   1736.6   1265.5
vp9_inv_dct_dct_16x16_sub8_add_neon:    2484.8   1828.7   2014.4   1506.5
vp9_inv_dct_dct_16x16_sub12_add_neon:   2851.2   2117.8   2294.8   1753.2
vp9_inv_dct_dct_16x16_sub16_add_neon:   3239.4   2408.3   2543.5   1994.9
vp9_inv_dct_dct_32x32_sub1_add_neon:     758.3    456.7    864.5    553.9
vp9_inv_dct_dct_32x32_sub2_add_neon:   10776.7   7949.8   8567.7   6819.7
vp9_inv_dct_dct_32x32_sub4_add_neon:   10865.6   8131.5   8589.6   6816.3
vp9_inv_dct_dct_32x32_sub8_add_neon:   12053.9   9271.3   9387.7   7564.0
vp9_inv_dct_dct_32x32_sub12_add_neon:  13328.3  10463.2  10217.0   8321.3
vp9_inv_dct_dct_32x32_sub16_add_neon:  14176.4  11509.5  11018.7   9062.3
vp9_inv_dct_dct_32x32_sub20_add_neon:  15301.5  12999.9  11855.1   9828.2
vp9_inv_dct_dct_32x32_sub24_add_neon:  16482.7  14931.5  12650.1  10575.0
vp9_inv_dct_dct_32x32_sub28_add_neon:  17589.5  15811.9  13482.8  11333.4
vp9_inv_dct_dct_32x32_sub32_add_neon:  18696.2  17049.2  14355.6  12089.7

After:
vp9_inv_dct_dct_16x16_sub1_add_neon:     273.0    189.5    211.7    235.8
vp9_inv_dct_dct_16x16_sub2_add_neon:    1203.5    998.2   1035.3    763.0
vp9_inv_dct_dct_16x16_sub4_add_neon:    1203.5    998.1   1035.5    760.8
vp9_inv_dct_dct_16x16_sub8_add_neon:    1926.1   1610.6   1722.1   1271.7
vp9_inv_dct_dct_16x16_sub12_add_neon:   2873.2   2129.7   2285.1   1757.3
vp9_inv_dct_dct_16x16_sub16_add_neon:   3221.4   2520.3   2557.6   2002.1
vp9_inv_dct_dct_32x32_sub1_add_neon:     753.0    457.5    866.6    554.6
vp9_inv_dct_dct_32x32_sub2_add_neon:    7554.6   5652.4   6048.4   4920.2
vp9_inv_dct_dct_32x32_sub4_add_neon:    7549.9   5685.0   6046.9   4925.7
vp9_inv_dct_dct_32x32_sub8_add_neon:    8336.9   6704.5   6604.0   5478.0
vp9_inv_dct_dct_32x32_sub12_add_neon:  10914.0   9777.2   9240.4   7416.9
vp9_inv_dct_dct_32x32_sub16_add_neon:  11859.2  11223.3   9966.3   8095.1
vp9_inv_dct_dct_32x32_sub20_add_neon:  15237.1  13029.4  11838.3   9829.4
vp9_inv_dct_dct_32x32_sub24_add_neon:  16293.2  14379.8  12644.9  10572.0
vp9_inv_dct_dct_32x32_sub28_add_neon:  17424.3  15734.7  13473.0  11326.9
vp9_inv_dct_dct_32x32_sub32_add_neon:  18531.3  17457.0  14298.6  12080.0

This is cherrypicked from libav commit
5eb5aec4.
Signed-off-by: Martin Storsjö <martin@martin.st>