[test] Avoid legacy PM default pipelines (O0,O1 etc) when running opt

Two lit tests were found running something like this:
  opt -O<n> -pass-locked-to-legacy-PM ...

The expand-atomicrmw-xchg-fp.ll seem to have used -O1 just to ensure
that the -atomic-expand pass were thinking that it wasn't running at
O0 level. Same thing can be ensured by using the -codegen-opt-level=1
option, making it possible to avoid using O1 in that test case.

In the vector-reductions-expanded.ll test case it was possible to
split the RUN line into using two opt invocations. First running
"opt -O2" using the new PM, and then running "opt -expand-reductions"
using the legacy PM.

I think that given this patch we get closer to removing code related
to 'AddOptimizationPasses' in opt.cpp.

Differential Revision: https://reviews.llvm.org/D137626