LoongArch: testsuite: Adapt widen-mul-rtx-cost-signed.c for r16-4949

Commit r16-4949 optimize mulw.d.w + srli.d to mulh.w,
so remove the check rule for mulw.d.w.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/widen-mul-rtx-cost-signed.c: Update.
1 file changed