dse: Use SUBREG_REG for copy_to_mode_reg in DSE replace_read for WORD_REGISTER_OPERAT...

dse: Use SUBREG_REG for copy_to_mode_reg in DSE replace_read for WORD_REGISTER_OPERATIONS targets [PR109040]

While we've agreed this is not the right fix for the PR109040 bug,
the patch clearly improves generated code (at least on the testcase from the
PR), so I'd like to propose this as optimization heuristics improvement
for GCC 14.

2023-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR target/109040
	* dse.cc (replace_read): If read_reg is a SUBREG of a word mode
	REG, for WORD_REGISTER_OPERATIONS copy SUBREG_REG of it into
	a new REG rather than the SUBREG.