Always assert in DAGCombine and not only when -debug is	enabled

Right now in DAG Combine check the validity of the returned type 
only when -debug is given on the command line. However usually 
the test cases in the validation does not use -debug. 
An Assert build should always check this.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224779 91177308-0d34-0410-b5e6-96231b3b80d8