fix Issue 14458 - very slow ubyte[] assignment (dmd doesn't use memset)

[WalterBright]

This should be about 4 to 8 times faster for larger byte counts.

Do OPmemcmp separately. Not going to do OPstrcpy or OPstrcmp as they are never generated by DMD.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	14458	normal	very slow ubyte[] assignment (dmd doesn't use memset)

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#11437"

[WalterBright]

@MartinNowak fyi