[DirectX] Specify NegZero as signed (#174840)
#171456 set `ImplicitTrunc` to false by default. So `NegZero` value was
no longer being created as a signed integer.
This caused a similar crash during `DXILIntrinsicExpansion` as reported
here:
https://github.com/llvm/llvm-project/pull/171456#issuecomment-3718690088.
This change fixes the test case from crashing in the DirectX backend by
manually specifying it as a signed integer.
