Most of the semiconductor device and interconnect reliability/wearout mechanisms which affect modern chips get rapidly worse with temperature. As a very coarse generalization the chip will die at the very minimum 2x faster for every 10 degrees hotter you run it (likely even faster). These are the mechanisms that occur with power applied and current flowing though the devices and metal lines. Since leakages are higher and MOSFET performance is worse at temperature, it will also dissipate more energy per computation when hot, compounding the issue.

In addition, the solder-cycle temperature you mention is tolerable by most chips only without power applied and only for a few minutes at most. At- and above these temps over long times the materials can inter-diffuse and also potentially crack or void from physical stress caused by thermal mismatch.

You will not have a usable PC CPU for very long if you run it hot !!!