fix issue 21696 - ice caused by partial extraction of basic ty

[ghost]

an assert failed in xmmstore() because it was passed a typenext.
The fix proposed is to extract the most nested type of the tnext chain.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21696	regression	DMD 2.095.1 Compilation Crash

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 "stable + dmd#12263"

[thewilsonator]

Otherwise looks good

[RazvanN7]

cc @WalterBright

[WalterBright]

see my comments