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

Author: frasercrmck

libclc/generic/lib/integer/abs_diff.inc

@@ -1,3 +1,5 @@
 _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;
 }