Add LLVM backend support for C For Loops
Merge pull request #291 from mppf/fix-llvm-c-for-2

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.

Reviewed by @ronawho

The LLVM backend seems to be working after this patch. I'll do more testing before the release.