[flang][nfc] Refine how output is defined in a test

Updates optimization-remark.f90. Makes sure that every RUN line:
* discords the actual output of the compilation (we only care about the
  optimisation remarks),
* re-uses the same definition of the output (better code re-use),
* doesn't generate object files - no need to use `-c` if `-emit-llvm` is
  sufficient.

Differential Revision: https://reviews.llvm.org/D158951