[PATCH] ARM64: start porting regression tests from AArch64

[Tim Northover]

Hi Ana,

> I thought the initial purpose of merging the AArch64 tests to ARM64 was to increase code coverage in ARM64.

Code coverage is good, but we really don't want to lose any areas
where AArch64 does better. What I'm doing is looking at each test and
deciding whether ARM64 is failing because of a deficiency or by random
chance.

If it's an easily fixed deficiency, I do so. If it's difficult, I file
a separate bug under the main merge issue (PR19392). The risk is tests
which are too imprecise: ones that will pass both AArch64 and ARM64,
but don't actually test for quality properly.

I don't have a solution for them, but by their nature they're
individual cases so they can be considered in parallel to the main
mere effort. I encourage anyone with an interest to make sure their
pet optimisations aren't lost!

Cheers.

Tim.