Improve coverage of ext-dce tests in risc-v testsuite

Improve coverage of ext-dce tests in risc-v testsuite

Inspired by Liao Shihua, this adjusts two tests in the RISC-V testsuite
to get more coverage.  Drop the -O1 argument and replace it with -fext-dce.
That way the test gets run across the full set of flags.  We just need to
make sure to skip -O0.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/core_list_init.c: Use -fext-dce rather than
	-O1.  Skip for -O0.
	* gcc.target/riscv/pr111384.c: Ditto.