[Bug target/121693] forgets to clear the padding bits when converting a _BitInt(1) on x86_64

[vincent-gcc at vinc17 dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121693

--- Comment #1 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Note: my initial testcase was with

  T v;
  volatile unsigned char c = 2;
  v.c = c;
  v.b = v.i;
  printf ("%d\n", (int) v.c);

(which should give 0, not 2). This could also be useful for the testsuite.