Fix SIMD12 GT_STORE_LCL_FLD

Fix SIMD12 GT_STORE_LCL_FLD

It was ignoring the local field offset, by checking for the wrong
operator.

This appeared as a recent regression, but I couldn't find any code
(including test) change that would have caused this.

The failure only happens for JitStress with remorphing stress. This
creates the STORE_LCL_FLD. Otherwise, it looks like we normally
end up with IND(LCL_FLD_ADDR). Maybe we should be morphing these
somewhere?

No x64 PMI asm diffs in corelib or the JIT\SIMD tree. With JitStress=1
there are exactly 6 asm diffs in the Vector3Interop test, which was the
one failing.

The same fix is made for both xarch and arm64 code generators.

Fixes #21935