[DAGCombiner] Add two calls to isVector before making calls to...

[DAGCombiner] Add two calls to isVector before making calls to getVectorElementType/getVectorNumElements to avoid an assert.

We looked through a BITCAST, but the bitcast might be a from a scalar type rather than a vector.

I don't have a test case. I stumbled onto it while prototyping another change that isn't ready yet.

llvm-svn: 325750