Addressing mode was not contained

[xtqqczze]

We should really just use MemoryMarshal.Write, but the codegen is suboptimal, see sharplab

Looks like a simple addressing mode was not contained. It's just that this PR replaces one unsafe code with another unsafe (a tiny bit better, but still). These APIs allocate, it is unlikely an extra lea makes perf difference here

@jakobbotsch do we have an issue tracking this?

Originally posted by @xtqqczze in #112616 (comment)

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[JulieLeeMSFT]

cc @dotnet/jit-contrib.

[jakobbotsch]

It turns out we did have an issue for this in #104538. Going to close as a duplicate.