[LegalizeDAG] Remove 'NeedInvert' code from expansion of BR_CC. Replace with an assert.

I accidentally triggered this code while doing some experiments and it doesn't look lke it could possibly work.

It calls 'getNOT' on a node that should be a CondCode.

I think to do this right we would need to swap the branch target and the fallthrough target. But that's not easy to do. Or we could create an explicit SetCC and feed that into a new BR_CC?

llvm-svn: 351022