[SelectionDAG] Fix noop detection for vectors in AssertZext/AssertSext in getNode

The assertion type is always supposed to be a scalar type. So if the result VT of the assertion is a vector, we need to get the scalar VT before we can compare them.

Similarly for the assert above it.

I don't have a test case because I don't know of any place we violate this today. A coworker found this while trying to use r347287 on the 6.0 branch without also having r336868

llvm-svn: 349390