[test] Fix aggregate-assign-call.c in preparation for -enable-npm-optnone

Pin the test to use -enable-npm-optnone.
Before, optnone wasn't implemented under NPM, so the LPM and NPM runs produced different IR. Now with -enable-npm-optnone, that is no longer necessary.

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D86008