Keep OBJ on mismatched struct assignment

[CarolEidt]

In a mismatched struct assignment, e.g. using Unsafe.As, we need to retain the OBJ(ADDR(lcl)) on the rhs of the assignment.

Fix #15237

[CarolEidt]

@benaadams - this incorporates your test case; I presume you're OK with that.
@dotnet/jit-contrib PTAL

There are zero jit-diffs for this change on Windows/x64 & x86. However, I plan to run desktop diffs on this as well before merging (I'm having some resource and other issues on my systems with desktop enlistments, so it might be a little while), since jit-diffs doesn't exercise Vector, as it uses crossgen.

[benaadams]

this incorporates your test case; I presume you're OK with that.

It doesn't cover the exact pattern I'd like to use, closest is

Vector4 q = Unsafe.ReadUnaligned<Vector4>(ref Unsafe.As<QuaternionStruct, byte>(ref this));

I'd probably like to use a straight ref conversion -> value

Vector4 q = Unsafe.As<QuaternionStruct, Vector4>(ref this);

An hope as a result q would be a SIMD register?

[benaadams]

Oh it does LengthSquaredUnsafeAs :)

[CarolEidt]

There are zero diffs for desktop x64 as well.

[briansull]

Issue #15906 is also fixed with this changeset, or alternatively broken without this changset