jumpless signed divide by 2

[WalterBright]

Because jumpless is better.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11441"

[UplinkCoder]

Seems to break unittests of mir and ocean.

[12345swordy]

What unit test in question that this PR break @UplinkCoder?

[UplinkCoder]

@12345swordy this: https://buildkite.com/dlang/dmd/builds/14374#60eed338-4991-4604-b3a7-a33abd597590
and this: https://buildkite.com/dlang/dmd/builds/14374#076124f8-2161-45d0-ab7d-f6cee445b65d

[WalterBright]

Found the problem, and added a test case for it so the test suite will catch it rather than buildkite.