RISC-V: Fix the riscv_legitimize_poly_move issue on targets where the minimal VLEN...

RISC-V: Fix the riscv_legitimize_poly_move issue on targets where the minimal VLEN exceeds 512.

riscv_legitimize_poly_move was expected to ensure the poly value is at most 32
times smaller than the minimal VLEN (32 being derived from '4096 / 128').
This assumption held when our mode modeling was not so precisely defined.
However, now that we have modeled the mode size according to the correct minimal
VLEN info, the size difference between different RVV modes can be up to 64
times. For instance, comparing RVVMF64BI and RVVMF1BI, the sizes are [1, 1]
versus [64, 64] respectively.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_legitimize_poly_move): Bump
	max_power to 64.
	* config/riscv/riscv.h (MAX_POLY_VARIANT): New.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/autovec/bug-01.C: New.
	* g++.target/riscv/rvv/rvv.exp: Add autovec folder.