Escape Analysis: Elide an array allocation in BitConverter.GetBytes pattern

[EgorBo]

JIT should be able to eliminate GC allocation in this snippet:

void Test(int value, Span<byte> dest) => 
    BitConverter.GetBytes(value).AsSpan(0, 3).CopyTo(dest);

This pattern could be used as a safe alternative to "write N-bytes of a primitive to a span of bytes". Alternative zero-alloc patterns don't look pretty/involve unsafe

Sub-tasks:

• Mark Memmove as non-escaping (JIT: Mark certain intrinsics as non-escaping #113093)
• Make sure BitConverter.GetBytes is inlineable (it is not currently) (JIT: boost inlining for methods that may return small arrays #114806)
• Escape Analysis for non-captured Span (JIT: enhance escape analysis to understand local struct fields #113977)
• Make this work in R2R as well
• Verify this works propery in Tier1+instr

cc @AndyAyersMS

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[AndyAyersMS]

Looks like my upcoming escape analysis changes will indeed stack allocate here...