[PATCH] Re-add calling emit_clobber in lower-subreg.cc's resolve_simple_move.

The previous patch:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=d8a6945c6ea22efa4d5e42fe1922d2b27953c8cd
aimed to eliminate redundant MOV instructions by removing calling
emit_clobber in lower-subreg.cc's resolve_simple_move.

First, I found that another patch address this issue:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=bdf2737cda53a83332db1a1a021653447b05a7e7
and even without removing calling emit_clobber,
the instruction generation is still as expected.

Second, removing the CLOBBER expression will have side effects.
When there is no CLOBBER expression and only SUBREG assignments exist,
according to the logic of the 'df_lr_bb_local_compute' function,
the register will be added to the basic block LR IN set.
This will cause the register's lifetime to span the entire function,
resulting in increased register pressure. Taking the newly added test case
'gcc/testsuite/gcc.target/riscv/pr43644.c' as an example,
removing the CLOBBER expression will lead to spill in some registers.

gcc/:
	* lower-subreg.cc (resolve_simple_move): Re-add calling emit_clobber
	immediately before moving a multi-word register by parts.

gcc/testsuite/:
	* gcc.target/riscv/pr43644.c: New test case.