[SelectionDAG] Constant fold (sext_inreg undef, VT) to 0 instead of undef.

The result of sign_extend_inreg needs to have as many sign bits
as requested by the VT argument. The easiest way to guarantee this
is to fold it to 0.

SystemZ test was modified to avoid using undef.

Fixes https://github.com/llvm/llvm-project/issues/55178

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D124696