patch: adding an assert to EvalLoad() in the Clang static analyzer

Hi Ted,

While fixing PR8419, I added an assert to catch errors in using
EvalLoad(). I didn't commit it as part of the fix, as I figured that
you prefer that each patch has a single purpose. I've created a
separate patch for the assert() and uploaded it to
Issue 3272041: add an assert() to catch errors in using EvalLoad() - Code Review -- would you please review it?

Please go ahead an add the assert.