We lowered JSConvertReceiver using JSToObject, w/o connecting the
JSToObject to the control chain (which is fine since that ToObject
cannot throw). But then the lowering of the JSToObject would insert
an IfSuccess, which is immediately dead, since it is not used. This
was fine in a sense that it didn't seem to crash anything, but we
now want to do more aggressive checking if IfSuccess/IfException
nodes and so we need to fix this sloppyness in the lowerings.

R=mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2228783003
Cr-Commit-Position: refs/heads/master@{#38484}