[test] Fix thinlto-debug-pm.c in preparation for -enable-npm-optnone

This fails due to the clang invocation running at -O0, producing an optnone function.
Then even with -O2 in the later invocations, LoopVectorizePass doesn't run on the optnone function.
So split this into an -O0 run and an -O2 run.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D86011