Int64Add is lowered to a new turbofan operator, Int32AddPair. The new
operator takes 4 inputs an generates 2 outputs. The inputs are the low
word of the left input, high word of the left input, the low word of the
right input, and high word of the right input. The ouputs are the low
and high word of the result of the addition.

R=titzer@chromium.org, v8-arm-ports@googlegroups.com

Review URL: https://codereview.chromium.org/1778493004

Cr-Commit-Position: refs/heads/master@{#34747}