There isn't any way to encode a variable in an SVE register, and there
isn't any way to encode a scalable offset, and as far as I know that's
unlikely to change in the near future. So suppress any debug info which
would require those encodings.
This isn't ideal, but we need to ship something which doesn't crash.
Alternatively, for Z registers, we could emit debug info assuming the
vector length is 128 bits, but that seems like it would lead to
unintuitive results.
The change to AArch64FrameLowering is needed to avoid a crash. But we
can't actually test that the returned offset is correct: LiveDebugValues
performs the query, then discards the result.
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