[SystemZ] Bugfix in SystemZTargetLowering::combineINT_TO_FP()

Make sure to also handle extended value types to avoid crashing.

Resulting integers greater than 64 bits are not optimized (i128 is not a
legal type), and vectorizing seems to result in libcalls instead of just
scalarization.

Other extended vector types like <10 x float> are however now handled and
should result in vectorized conversions.

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D125881