Add LLVM backend support for C For Loops

Directly support C for loops in the LLVM backend. Tested with make check for LLVM and non-llvm builds and with test/types/range/diten/cForLoops and with zipRange.chpl therein with --no-scalar-replacement and/or --no-inline. Tests OK.

Also fixes up some previous LLVM code generation that added unnecessary ICmpNE in some cases.
Nothing you need to change -- but I'll update this with a real explanation soon.