Convert the global variable IC and the array-length fast path to
named temps. These three handlers are large -- GetGlobal and
SetGlobal carry ~20 named values across each path (realm pointer,
global object, declarative environment, cache pointer, two serial
numbers, the shape, the cached shape, the dictionary generations,
the property offset, the named-properties pointer, the loaded value
and tag, plus environment-binding state) -- and the migration makes
those values easy to read instead of having to remember which slot
each piece of state was occupying.
GetLength's "magical" int-to-double widening picks up a named
temp for the sign-bit check and the result, which removes another
"NB: load_operand clobbers t0" comment.
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