target/arm: Merge arm_cpu_vq_map_next_smaller into sole caller

Coverity reports, in sve_zcr_get_valid_len,

"Subtract operation overflows on operands
arm_cpu_vq_map_next_smaller(cpu, start_vq + 1U) and 1U"

First, the aarch32 stub version of arm_cpu_vq_map_next_smaller,
returning 0, does exactly what Coverity reports.  Remove it.

Second, the aarch64 version of arm_cpu_vq_map_next_smaller has
a set of asserts, but they don't cover the case in question.
Further, there is a fair amount of extra arithmetic needed to
convert from the 0-based zcr register, to the 1-base vq form,
to the 0-based bitmap, and back again.  This can be simplified
by leaving the value in the 0-based form.

Finally, use test_bit to simplify the common case, where the
length in the zcr registers is in fact a supported length.

Reported-by: Coverity (CID 1407217)
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20191118091414.19440-1-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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