Original commit message:
    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=ahaas@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1807013002

Cr-Commit-Position: refs/heads/master@{#34840}