asf: Fix calling of emit_move_insn on registers of different modes [PR119884]

asf: Fix calling of emit_move_insn on registers of different modes [PR119884]

This patch uses `lowpart_subreg` for the base register initialization,
instead of zero-extending it. We had tried this solution before, but
we were leaving undefined bytes in the upper part of the register.
This shouldn't be happening as we are supposed to write the whole
register when the load is eliminated. This was occurring when having
multiple stores with the same offset as the load, generating a
register move for all of them, overwriting the bit inserts that were
inserted before them.

In order to overcome this, we are removing redundant stores from the
sequence, i.e. stores that write to addresses that will be overwritten
by stores that come after them in the sequence. We are using the same
bitmap that is used for the load elimination check, to keep track of
the bytes that are written by each store.

Also, we are now allowing the load to be eliminated even when there
are overlaps between the stores, as there is no obvious reason why we
shouldn't do that, we just want the stores to cover all of the load's
bytes.

Bootstrapped/regtested on AArch64 and x86_64.

	PR rtl-optimization/119884

gcc/ChangeLog:

	* avoid-store-forwarding.cc (process_store_forwarding):
	Use `lowpart_subreg` for the base register initialization
	and remove redundant stores from the store/load sequence.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr119884.c: New test.