lower-bitint: Fix lowering of middle sized _BitInt operations which can throw [PR112770]

The middle kind _BitInt lowering is mostly done by casting the BITINT_TYPE
operands (if any) to a signed/unsigned integer type which has larger/equal
precision, using such integer type also for the lhs (if BITINT_TYPE) and
and adding a cast after the statement from that new lhs to the old
(BITINT_TYPE) lhs.  Note, for middle kind this isn't done for GIMPLE_CALLs.
Most of the time that works nicely, the exception as the following testcase
shows is -fnon-call-exceptions and some operations which can trap.  Because
inserting the cast to a new lhs after the statement results in a trapping
statement in the middle of a basic block.
The following patch fixes that by emitting the cast on the fallthru edge
instead.

2023-12-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/112770
	* gimple-lower-bitint.cc (gimple_lower_bitint): When adjusting
	lhs of middle _BitInt setter which ends bb, insert cast on
	the fallthru edge rather than after stmt.

	* gcc.dg/bitint-45.c: New test.