simplify-rtx: Fix up shift/rotate VOIDmode count handling [PR123523]

The following testcase ICEs on i686-linux, because the HW in that
case implements the shift as shifting by 64-bit count (anything larger
or equal to number of bits in the first operand's element results
in 0 or sign copies), so the machine description implements it as
such as well.
Now, because shifts/rotates can have different modes on the first
and second operand, when the second one has VOIDmode (i.e. CONST_INT,
I think CONST_WIDE_INT has non-VOIDmode and CONST_DOUBLE with VOIDmode
is hopefully very rarely used), we need to choose some mode for the
wide_int conversion.  And so far we've been choosing BITS_PER_WORD/word_mode
or the mode of the first operand's element, whichever is wider.
That works fine on 64-bit targets, CONST_INT has always at most 64 bits,
but for 32-bit targets uses SImode.

Because HOST_BITS_PER_WIDE_INT is always 64, the following patch just
uses that plus DImode instead of BITS_PER_WORD and word_mode.

2026-01-12  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/123523
	* simplify-rtx.cc (simplify_const_binary_operation): Use
	DImode for VOIDmode shift and truncation counts if int_mode
	is narrower than HOST_BITS_PER_WIDE_INT rather than
	word_mode if int_mode it is narrower than BITS_PER_WORD.

	* gcc.target/i386/pr123523.c: New test.

(cherry picked from commit dd8df074c9ceaa723e82f06bef8714f46ecccb18)