vtn/opencl: add clz support

This is needed for OpenCL

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_builtin_builder.h

    return nir_bcsel(b, nir_ieq(b, s, nir_imm_intN_t(b, 0, s->bit_size)), x, y);
 }
 
+static inline nir_ssa_def *
+nir_clz_u(nir_builder *b, nir_ssa_def *a)
+{
+   nir_ssa_def *val;
+   val = nir_isub(b, nir_imm_intN_t(b, a->bit_size - 1, 32), nir_ufind_msb(b, a));
+   return nir_u2u(b, val, a->bit_size);
+}
+
 #ifdef __cplusplus
 }
 #endif

src/compiler/spirv/vtn_opencl.c

       return nir_rotate(nb, srcs[0], srcs[1]);
    case OpenCLstd_Smoothstep:
       return nir_smoothstep(nb, srcs[0], srcs[1], srcs[2]);
+   case OpenCLstd_Clz:
+      return nir_clz_u(nb, srcs[0]);
    case OpenCLstd_Select:
       return nir_select(nb, srcs[0], srcs[1], srcs[2]);
    case OpenCLstd_Step: