Use MVT instead of EVT in LowerVECTOR_SHUFFLEtoBlend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173009 91177308-0d34-0410-b5e6-96231b3b80d8
Use <0 checks in place of ==-1 because it results in simpler code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173010 91177308-0d34-0410-b5e6-96231b3b80d8