[committed][RISC-V][PR target/123274] Add missing condition in usmul<mode>3 pattern

[committed][RISC-V][PR target/123274] Add missing condition in usmul<mode>3 pattern

As Andrew P. noted in the BZ, the expander is missing elements in its condition
leading to generation of an insn that can't be matched.

This adds the necessary condition to the usmul<mode>3 expander which in turn
fixes the ICE.  I just checked and that expander wansn't in gcc-15, so this is
just a gcc-16 issue.

Tested on riscv32-elf and riscv64-elf.  I have a bootstrap in flight on the
Pioneer, but I'm not expecting any surprises.  Much like the patch earlier
today, I'm going to push this now rather than wait for pre-commit CI.

	PR target/123274
gcc/
	* config/riscv/riscv.md (usmul<mode>3): Add proper condition.

gcc/testsuite/
	* gcc.target/riscv/pr123274.c: New test.