bitint: Fix bitfield loads in handle_cast [PR114433]

bitint: Fix bitfield loads in handle_cast [PR114433]

We ICE on the following testcase, because handle_cast was incorrectly
testing !m_first to see whether it should use m_data[m_bitfld_load + 1]
or fresh SSA_NAME for a PHI result.
Now, m_first is in the routine sometimes temporarily cleared in between
doing prepare_data_in_out and the !m_first check and only before returning
restored from the save_first copy.
Without this patch, we try to use the same SSA_NAME (_12 here) in 2
different PHI results which is obviously invalid IL and ICEs very quickly.

2024-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/114433
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast): For
	m_bitfld_load check save_first rather than m_first.

	* gcc.dg/torture/bitint-68.c: New test.