[flang] Resolve "possible performance problem" issue spam (#79769)

Four "issues" on GitHub report possible performance problems, likely
detected by static analysis. None of them would ever make a measureable
difference in compilation time, but I'm resolving them to clean up the
open issues list.

Fixes https://github.com/llvm/llvm-project/issues/79703, .../79705,
.../79706, & .../79707.