RISC-V: Use biggest_mode as mode for constants.

In compute_nregs_for_mode we expect that the current variable's mode is
at most as large as the biggest mode to be used for vectorization.

This might not be true for constants as they don't actually have a mode.
In that case, just use the biggest mode so max_number_of_live_regs
returns 1.

This fixes several test cases in the test suite.

gcc/ChangeLog:

	PR target/116655

	* config/riscv/riscv-vector-costs.cc (max_number_of_live_regs):
	Use biggest mode instead of constant's saved mode.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr116655.c: New test.