[cleanup] Nuke the 'VectorOp' bit of the promote method names.

This doesn't add any information for methods in the VectorLegalizer
class that clearly take SDAG operations to legalize.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212157 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] Hoist the promotion dispatch logic into the promote function
so that we can use return to express it more cleanly and avoid so many
nested switch statements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212158 91177308-0d34-0410-b5e6-96231b3b80d8