Fix issue 18034 - Bad codegen for OPvecfill

[LemonBoy]

Do not replace vector values if that implies a change of Ety, doing so
may end up initializing the vector with the wrong content.

Another idea would be to special-case OPvecfill in the Ety assignment below to take advantage of the value replacement.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18034	major	SIMD optimization issues

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 "master + dmd#7994"

[JinShil]

Welcome back!

[wilzbach]

Should this have // REQUIRED_ARGS: -O or maybe both variants with ARG_SETS?

[LemonBoy]

Good catch, I forgot about the -O.

[JinShil]

Assigning to @WalterBright Oops. Already done. Ping! then.

[ibuclaw]

This PR introduced a regression https://issues.dlang.org/show_bug.cgi?id=23084