[SystemZ] Use VT (not ArgVT) for SlotVT in LowerCall(). (#82475)

When an integer argument is promoted and *not* split (like i72 -> i128 on
a new machine with vector support), the SlotVT should be i128, which is
stored in VT - not ArgVT.

Fixes #81417

(cherry picked from commit 9c0e45d7)