RISC-V: Fix factor in dwarf_poly_indeterminate_value [PR116305]

RISC-V: Fix factor in dwarf_poly_indeterminate_value [PR116305]

This patch is to fix the bug (BugId:116305) introduced by the commit
bd93ef for risc-v target.

The commit bd93ef changes the chunk_num from 1 to TARGET_MIN_VLEN/128
if TARGET_MIN_VLEN is larger than 128 in riscv_convert_vector_bits. So
it changes the value of BYTES_PER_RISCV_VECTOR. For example, before
merging the commit bd93ef and if TARGET_MIN_VLEN is 256, the value
of BYTES_PER_RISCV_VECTOR should be [8, 8], but now [16, 16]. The value
of riscv_bytes_per_vector_chunk and BYTES_PER_RISCV_VECTOR are no longer
equal.

Prologue will use BYTES_PER_RISCV_VECTOR.coeffs[1] to estimate the vlenb
register value in riscv_legitimize_poly_move, and dwarf2cfi will also
get the estimated vlenb register value in riscv_dwarf_poly_indeterminate_value
to calculate the number of times to multiply the vlenb register value.

So need to change the factor from riscv_bytes_per_vector_chunk to
BYTES_PER_RISCV_VECTOR, otherwise we will get the incorrect dwarf
information. The incorrect example as follow:

```
csrr    t0,vlenb
slli    t1,t0,1
sub     sp,sp,t1

.cfi_escape 0xf,0xb,0x72,0,0x92,0xa2,0x38,0,0x34,0x1e,0x23,0x50,0x22
```

The sequence '0x92,0xa2,0x38,0' means the vlenb register, '0x34' means
the literal 4, '0x1e' means the multiply operation. But in fact, the
vlenb register value just need to multiply the literal 2.

	PR target/116305

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_dwarf_poly_indeterminate_value): Take
	BYTES_PER_RISCV_VECTOR for *factor instead of riscv_bytes_per_vector_chunk.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/scalable_vector_cfi.c: New test.

Signed-off-by: Zhijin Zeng <zhijin.zeng@spacemit.com>