RISC-V: Fix natural regsize for fixed-vlmax of -march=rv64gc_zve32f

RISC-V: Fix natural regsize for fixed-vlmax of -march=rv64gc_zve32f

This patch fixes 12 ICEs of "full coverage" testing:
Running target riscv-sim/-march=rv64gc_zve32f/-mabi=lp64d/-mcmodel=medlow/--param=riscv-autovec-lmul=dynamic/--param=riscv-autovec-preference=fixed-vlmax
FAIL: gcc.dg/torture/pr96513.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr96513.c   -O3 -g  (internal compiler error: Segmentation fault)

Running target riscv-sim/-march=rv64gc_zve32f/-mabi=lp64d/-mcmodel=medlow/--param=riscv-autovec-lmul=m4/--param=riscv-autovec-preference=fixed-vlmax
FAIL: gcc.dg/torture/pr111048.c   -O2  (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr111048.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr111048.c   -O3 -g  (internal compiler error: Segmentation fault)

FAIL: gcc.dg/torture/pr96513.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr96513.c   -O3 -g  (internal compiler error: Segmentation fault)

Running target riscv-sim/-march=rv64gc_zve32f/-mabi=lp64d/-mcmodel=medlow/--param=riscv-autovec-lmul=m8/--param=riscv-autovec-preference=fixed-vlmax
FAIL: gcc.dg/torture/pr96513.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  (internal compiler error: Segmentation fault)
FAIL: gcc.dg/torture/pr96513.c   -O3 -g  (internal compiler error: Segmentation fault)

Running target riscv-sim/-march=rv64gc_zve32f/-mabi=lp64d/-mcmodel=medlow/--param=riscv-autovec-preference=fixed-vlmax
FAIL: gcc.c-torture/execute/20000801-1.c   -O2  (internal compiler error: Segmentation fault)
FAIL: gcc.c-torture/execute/20000801-1.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  (internal compiler error: Segmentation fault)
FAIL: gcc.c-torture/execute/20000801-1.c   -O3 -g  (internal compiler error: Segmentation fault)

The root cause of those ICEs is vector register size = 32bits, wheras scalar register size = 64bit.
That is, vector regsize < scalar regsize on -march=rv64gc_zve32f FIXED-VLMAX.

So the original natural regsize using scalar register size is incorrect. Instead, we should return minimum regsize between vector regsize and scalar regsize.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_regmode_natural_size): Fix ICE for
	FIXED-VLMAX of -march=rv32gc_zve32f.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/bug-4.c: New test.
	* gcc.target/riscv/rvv/autovec/bug-5.c: New test.
	* gcc.target/riscv/rvv/autovec/bug-6.c: New test.