bitintlower: Fix up ?ROTATE_EXPR lowering [PR117847]

bitintlower: Fix up ?ROTATE_EXPR lowering [PR117847]

In the ?ROTATE_EXPR lowering I forgot to handle rotation by 0 correctly.
INTEGER_CST 0 is very unlikely, it would be probably folded away, but
a non-constant count can't use just p - n because then the shift count
is out of bounds for zero.

In the FE I use n == 0 ? x : (x << n) | (x >> (p - n)) but bitintlower
here isn't prepared at this point to have bb split and am not sure if
using COND_EXPR is a good idea either, so the patch uses (p - n) % p.
Perhaps I should just disable lowering the rotate in the FE for the
non-mode precision BITINT_TYPEs too.

2024-12-03  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/117847
	* gimple-lower-bitint.cc (gimple_lower_bitint) <case LROTATE_EXPR>:
	Use m = (p - n) % p instead of m = p - n for the other shift count.

	* gcc.dg/torture/bitint-75.c: New test.