LiveIntervals: Remove assertion

This testcase is invalid, and caught by the verifier. For the verifier
to catch it, the live interval computation needs to complete. Remove
the assert so the verifier catches this, which is less confusing.

In this testcase there is an undefined use of a subregister, and lanes
which aren't used or defined. An equivalent testcase with the
super-register shrunk to have no untouched lanes already hit this
verifier error.

llvm-svn: 371792