Optimizations - Programmer vs. Compiler?
Amen! If you're not already sweating optimization (i.e., you've got some supremely high-performance code), then it's better to write straightforward code. Especially with C, which has pretty mature compilers.
Another thing to consider is that compilers for a modern RISC architecture have pretty intense optimization built in just to handle the instruction scheduling (re-ordering instructions to avoid pipeline stalls, etc.) that any trivial optimizations you might make would be "lost in the noise" anyway.
That said, the big optimizations will always be worthwhile: cacheing results so you don't have to read from a file/database again, using lazy initialization to avoid populating data structures you may not use, validating inputs so you don't get halfway through an expensive operation and then have to roll back the transaction and throw an error, etc. But moving loop invariants? Maybe in a new language with an immature compiler, or a scripting language (just how efficient is PHP, anyway? Python?), but any modern compiler will make that irrelevant.