TypedArrayToSorted calls TypedArraySort, which in turn calls MergeSortTypedArray. MergeSortTypedArray creates another copy of the TypedArray, which isn't actually necessary for TypedArrayToSorted. The same issue applies for ArrayToSorted.

Change MergeSortTypedArray to return the sorted array. TypedArraySort now
needs to copy the sorted elements back into the original array and
TypedArrayToSorted can directly return the sorted array. When TypedArrayToSorted
is called without a comparator function, we still have to create a copy in
TypedArrayToSorted which is then passed to TypedArrayNativeSort. When the
input has fewer than two elements, we only have to copy the input without performing
any sort operations.

Depends on D154261

We generally prefer single-line comments in new code.

Depends on D154262

Inline ArraySort into ArrayToSorted in preparation for the next part.

Depends on D154263

Change MergeSort to directly modify its input instead of creating a copy.
ArraySort now needs to copy the input and re-arrange any holes, whereas
ArrayToSorted can directly return the result from MergeSort.

Depends on D154264

Backed out for causing SM assertion failures on js/Value.h.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=a8f802732daf1549c6ba608d0a5d2c30034f661c&selectedTaskRun=LAEWMEHjS8mmCCo0R45Afg.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=387128882&repo=autoland

Backout link: https://hg.mozilla.org/integration/autoland/rev/66e3220110ba0dd99ba7d45684ac4731886a59a9

https://hg.mozilla.org/mozilla-central/rev/aef2c47ad1c4
https://hg.mozilla.org/mozilla-central/rev/8ea5622a4264
https://hg.mozilla.org/mozilla-central/rev/61db7e2983a3
https://hg.mozilla.org/mozilla-central/rev/f98b7c864bea
https://hg.mozilla.org/mozilla-central/rev/98f1a8e535e9