NFCI: Remove VF argument from isScalarWithPredication

As discussed in D102437, the VF argument to isScalarWithPredication
seems redundant, so this is intended to be a non-functional change. It
seems wrong to query the widening decision at this point. Removing the
operand and code to get the widening decision causes no unit/regression
tests to fail. I've also found no issues running the LLVM test-suite.

This subsequently removes the VF argument from isPredicatedInst as well,
since it is no longer required.