aarch64: Use the MUL VL form of SVE PRF[BHWD]

aarch64: Use the MUL VL form of SVE PRF[BHWD]

The expansions of the svprf[bhwd] instructions weren't taking
advantage of the immediate addressing mode.

gcc/
	* config/aarch64/aarch64.c (offset_6bit_signed_scaled_p): New function.
	(offset_6bit_unsigned_scaled_p): Fix typo in comment.
	(aarch64_sve_prefetch_operand_p): Accept MUL VLs in the range
	[-32, 31].

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/asm/prfb.c: Test for a MUL VL range of
	[-32, 31].
	* gcc.target/aarch64/sve/acle/asm/prfh.c: Likewise.
	* gcc.target/aarch64/sve/acle/asm/prfw.c: Likewise.
	* gcc.target/aarch64/sve/acle/asm/prfd.c: Likewise.