expr: Fix REDUCE_BIT_FIELD for constants [PR95694]

expr: Fix REDUCE_BIT_FIELD for constants [PR95694]

This is yet another PR caused by constant integer rtxes not storing
a mode.  We were calling REDUCE_BIT_FIELD on a constant integer that
didn't fit in poly_int64, and then tripped the as_a<scalar_int_mode>
assert on VOIDmode.

AFAICT REDUCE_BIT_FIELD is always passed rtxes that have TYPE_MODE
(rather than some other mode) and it just fills in the redundant
sign bits of that TYPE_MODE value.  So it should be safe to get
the mode from the type instead of the rtx.  The patch does that
and asserts that the modes agree, where information is available.

That on its own is enough to fix the bug, but we might as well
extend the folding case to all constant integers, not just those
that fit poly_int64.

gcc/
	PR middle-end/95694
	* expr.c (expand_expr_real_2): Get the mode from the type rather
	than the rtx, and assert that it is consistent with the mode of
	the rtx (where known).  Optimize all constant integers, not just
	those that can be represented in poly_int64.

gcc/testsuite/
	PR middle-end/95694
	* gcc.dg/pr95694.c: New test.