The wrong placement of add pass with optimizations led to...

The wrong placement of add pass with optimizations led to -funique-internal-linkage-names being disabled.

Fixed the placement of the MPM.addpass for UniqueInternalLinkageNames to make
it work correctly with -O2 and new pass manager. Updated the tests to
explicitly check O0 and O2.

Previously, the addPass was placed before BackendUtil.cpp#L1373 which is wrong
as MPM gets assigned at this point and any additions to the pass vector before
this is wrong. This change just moves it after MPM is assigned and places it at
a point where O0 and O0+ can share it.

Differential Revision: https://reviews.llvm.org/D87921