Don't preinitialize fields that are never read

[MichalStrehovsky]

• Track whether a field is accessed outside preinitialized static initializers
• If it's not accessed, emit it as zero-initialized

Draft for rt-sz measurements. Not clear if this will have savings since RyuJIT is already pretty good at inlining the values of fields, which means we often eliminate entire static bases already.

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[MichalStrehovsky]

rt-sz says not worth it!