[FastISel] Remove an performance debugging assert.

As Jim pointed out this assert isn't really needed to test for correctness,
because the code right afterwards does the same check and falls-back to
SelectionDAG - as intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215735 91177308-0d34-0410-b5e6-96231b3b80d8