Merge computed MemberExpression in member chain

[azz]

// Before
data
  [key]('foo')
  .then(() => console.log('bar'))
  .catch(() => console.log('baz'));

// After
data[key]('foo')
  .then(() => console.log('bar'))
  .catch(() => console.log('baz'));

Fixes #1313

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I'm not 100% sure if it's worth adding a special case for computeds here though? This issue is the second-highest upvoted bug though.

[vjeux]

It doesn't hurt :)