[flang][NFC] Speed up large DATA statement initializations (#67585)

To ensure that the map from symbols to their initial images has an entry
for a particular symbol, use std::map<>::find() before
std::map<>::emplace() to avoid needless memory allocation and
deallocation. Also, combine adjacent intervals in the lists of
initialized ranges so that contiguous initializations don't require long
lists.

Fixes https://github.com/llvm/llvm-project/issues/66452.