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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@370613 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm