fix Issue 20466 - Optimizer clobbering msw register when testing it f…

[WalterBright]

…or 0

Blocking #10679

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20466	critical	Optimizer clobbering msw register when testing it for 0

Testing this PR locally

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

dub fetch digger
dub run digger -- build "stable + dmd#10696"

[thewilsonator]

Should this target stable.

[WalterBright]

If it passes :-)

[WalterBright]

Refactored #10679 so this won't block it.

[WalterBright]

Note: this is an ugly little goto I added. It should be refactored. But it is better to separate the fix from the refactor, which is why I did it this way. It's good that D supports goto to enable this sort of thing.

[thewilsonator]

Indeed, can you come up with a slightly more descriptive name than L1? Otherwise looks great.

[WalterBright]

The comment associated with it is clear enough. Rebased to stable.

[WalterBright]

The buildkite errors:

dmd/complex.d(57): Deprecation: `opMul` is deprecated.  Use `opBinary(string op)(...) if (op == "*")` instead.
--
  | dmd/complex.d(57): Deprecation: `opMul_r` is deprecated.  Use `opBinaryRight(string op)(...) if (op == "*")` instead.
  | dmd/complex.d(62): Deprecation: `opMul` is deprecated.  Use `opBinary(string op)(...) if (op == "*")` instead.
  | dmd/complex.d(62): Deprecation: `opMul_r` is deprecated.  Use `opBinaryRight(string op)(...) if (op == "*")` instead.
  | dmd/complex.d(67): Deprecation: `opDiv` is deprecated.  Use `opBinary(string op)(...) if (op == "/")` instead.

are not relevant to this PR.