Add AVX versions of blend vector operations and fix some issues noticed

in Nadav's r139285 and r139287 commits.

1) Rename vsel.ll to a more descriptive name
2) Change the order of BLEND operands to "Op1, Op2, Cond", this is
necessary because PBLENDVB is already used in different places with
this order, and it was being emitted in the wrong way for vselect
3) Add AVX patterns and tests for the same SSE41 instructions

llvm-svn: 139305