libclc: Fix signed integer underflow in abs_diff

We noticed this same issue in our own implementation of abs_diff, and
the same issue also came up in the abs_diff reference function in the
OpenCL CTS.

Reviewed By: rjodinchr

Differential Revision: https://reviews.llvm.org/D159275

GitOrigin-RevId: 37a3de1e2eedf848b8442217ef3790436f69a7db
1 file changed