[SystemZ] Use the EVT version of getVectorVT() in combineTruncateExtract(). (#100150)
A test case showed up where the new vector type is v24i16, which is not a simple
MVT. In order to get an extended value type for cases like this, EVT::getVectorVT()
needs to be called instead of MVT::getVectorVT(), otherwise the following call
to getVectorElementType() in combineExtract() will fail.