noiseReduction: allow separate strengths to be specified for intra and inter CUs
analysis: add chroma distortion to rdLevels 3 and 4

At these rdLevels, inter/bidir and merge candidate decisions were being taken
based on luma sa8dCost only. This will increase bitrate and lower ssim slightly,
with better subjective quality.

Also fixed some naming nits.
analysis: cache m_bChromaSa8d and reduce redundant work

Renamed some 'part' variables to 'puIdx' to avoid variable shadow warnings and
for consistency with search.cpp
asm: chroma_vpp[4x4] for colorspace i420 in avx2: improve 228c->184c