[AArch64] Add option -msve-streaming-vector-bits= . (#144611)
This is similar to -msve-vector-bits, but for streaming mode: it
constrains the legal values of "vscale", allowing optimizations based on
that constraint.
This also fixes conversions between SVE vectors and fixed-width vectors
in streaming functions with -msve-vector-bits and
-msve-streaming-vector-bits.
This rejects any use of arm_sve_vector_bits types in streaming
functions; if it becomes relevant, we could add
arm_sve_streaming_vector_bits types in the future.
This doesn't touch the __ARM_FEATURE_SVE_BITS define.