[SystemZ] Avoid scalarization of [SU]INT_TO_FP ISD-nodes.

The type legalizer will scalarize vector conversions from integer to floating
point if the source element size is less than that of the result.

This is avoided now by inserting a zero/sign-extension of the source vector
before type legalization.

Review: Ulrich Weigand

Differential revision: https://reviews.llvm.org/D75978