Merge `LOAD_METHOD` back into `LOAD_ATTR` for better specialization

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LOAD_METHOD split off from LOAD_ATTR in 3.7 as a compiler based optimisation.

I proposed to merge it back to enable better specialization. The actual LOAD_METHOD instruction is left around as a virtual instruction at the moment.

See faster-cpython/ideas#400 (reply in thread)

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Reopening to remind myself to complete docs.