change get_best_mode args int -> HOST_WIDE_INT [PR121264]

The following testcase is miscompiled, because byte 0x20000000
is bit 0x100000000 and ifcombine incorrectly combines the two loads
into a BIT_FIELD_REF even when they are very far away.
The problem is that gimple-fold.cc ifcombine uses get_best_mode heavily,
and that function has just int bitsize and int bitpos arguments, so
when called e.g. with
  if (get_best_mode (end_bit - first_bit, first_bit, 0, ll_end_region,
                     ll_align, BITS_PER_WORD, volatilep, &lnmode))
where end_bit - first_bit doesn't fit into int, it is silently truncated.
If there was just a single problematic get_best_mode call, I would probably
just check for overflows in the caller, but there are many.
And the two arguments are used solely as arguments to
bit_field_mode_iterator constructor which has HOST_WIDE_INT arguments,
so I think the easiest fix is just make the get_best_mode arguments
also HOST_WIDE_INT.

2025-07-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/121264
	* machmode.h (get_best_mode): Change type of first 2 arguments
	from int to HOST_WIDE_INT.
	* stor-layout.cc (get_best_mode): Likewise.

	* gcc.dg/tree-ssa/pr121264.c: New test.

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