[x86] Switch the blend implementation to use a MVT switch rather than
awkward conditions. The readability improvement of this will be even
more important as I generalize it to handle more types.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218205 91177308-0d34-0410-b5e6-96231b3b80d8