shader: use uint comparison for UCMP

Fixes a few piglit tests, such as:
spec/glsl-1.30/execution/switch/fs-default_last

src/vrend_shader.c

@@ -1718,7 +1718,7 @@ iter_instruction(struct tgsi_iterate_context *iter,
        EMIT_BUF_WITH_RET(ctx, buf);
        break;
    case TGSI_OPCODE_UCMP:
-      snprintf(buf, 255, "%s = mix(%s, %s, notEqual(%s, vec4(0.0)))%s;\n", dsts[0], srcs[2], srcs[1], srcs[0], writemask);
+      snprintf(buf, 255, "%s = mix(%s, %s, notEqual(floatBitsToUint(%s), uvec4(0.0)))%s;\n", dsts[0], srcs[2], srcs[1], srcs[0], writemask);
       EMIT_BUF_WITH_RET(ctx, buf);
       break;
    case TGSI_OPCODE_END: