[X86][AVX] Rename + cleanup lowerShuffleAsLanePermuteAndBlend. NFCI.

Rename to lowerShuffleAsLanePermuteAndShuffle to make it clear that not just blends are performed.

Cleanup the in-lane shuffle mask generation to make it more obvious what's going on.

Some prep work noticed while investigating the poor shuffle code mentioned in D66004.

llvm-svn: 370613