libclc: Fix signed integer underflow in abs_diff

[frasercrmck]

Summary

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.

[rjodinchr]

clspv is also implementing abs_diff like that, and it is OpenCL compliant:
https://github.com/google/clspv/blob/b14ef325ef31894b0aaeca9a84e65b10ae53a816/lib/ReplaceOpenCLBuiltinPass.cpp#L1148
https://godbolt.org/z/x7vzd85os

libclc/generic/lib/integer/abs_diff.inc

 _CLC_OVERLOAD _CLC_DEF __CLC_U_GENTYPE abs_diff(__CLC_GENTYPE x, __CLC_GENTYPE y) {
-  return __builtin_astype((__CLC_GENTYPE)(x > y ? x-y : y-x), __CLC_U_GENTYPE);
+  __CLC_U_GENTYPE ux = __builtin_astype(x, __CLC_U_GENTYPE);
+  __CLC_U_GENTYPE uy = __builtin_astype(y, __CLC_U_GENTYPE);
+  return x > y ? ux - uy : uy - ux;
 }