[PGO] Preserve analysis results when nothing was instrumented

The 'PGOInstrumentationGen' pass should preserve all analysis results
when nothing was actually instrumented. Currently, only modules that
contain at least a single function definition are instrumented. When a
module contains only function declarations and, optionally, global
variable definitions (a module for the regular-LTO phase for thin-LTO
when LTOUnit splitting is enabled, for example), such module is not
instrumented (yet?) and there is no reason to invalidate any analysis
results.

NFC.