Re: suspicious valgrind reports about radixtree/tidstore on arm64

I wrote:
> I hypothesize that the reason we're not seeing equivalent failures
> on x86_64 is one of

> 1. x86_64 valgrind is stupider than aarch64, and fails to track that
> the contents of the SIMD registers are only partially defined.

> 2. x86_64 valgrind is smarter than aarch64, and is able to see
> that the "mask off invalid entries" step removes all the
> potentially-uninitialized bits.

Hah: it's the second case.  If I patch radixtree.h as attached,
then x86_64 valgrind complains about

==00:00:00:32.759 247596== Conditional jump or move depends on uninitialised value(s)
==00:00:00:32.759 247596==    at 0x52F668: local_ts_node_16_search_eq (radixtree.h:1018)

showing that it knows that the result of vector8_highbit_mask is
only partly defined.