fix Issue 23218 - cgxmm.d:1373 assert fail

[WalterBright]

This was actually two bugs:

1. the vecfill problem outlined in the bug report
2. a type paint was being used where a cast should have been

Also adjusted the strings emitted by the debug output so they matched SIMD types.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23218	major	ICE: src/dmd/backend/cgxmm.d:1373: Assertion `0' failed.

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#14790"

[ibuclaw]

Should target stable.

[WalterBright]

@RazvanN7 this only fixes 23218, not all those other issues.

[RazvanN7]

@WalterBright I rebased this on stable for you, but you probably pushed from your master branch.