[clang][NFC] Extract EmitAssemblyHelper::shouldEmitRegularLTOSummary

The code to check if the regular LTO summary should be emitted and to
add the corresponding module flags was duplicated in the
'EmitAssemblyHelper::EmitAssemblyWithLegacyPassManager' and
'EmitAssemblyHelper::RunOptimizationPipeline' methods.

In order to eliminate these code duplications, the
'EmitAssemblyHelper::shouldEmitRegularLTOSummary' method has been
extracted. The method returns a bool value, the value is 'true' if the
module summary should be emitted. The patch keeps the setting of the
module flags inline.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D123026