Clarify SCEV comments.

We handle for(i=n; i>0; i -= s) by canonicalizing within SCEV to for(i=-n; i<0; i += s).
Change llvm-cov output formatting to be more similar to gcov.

- Replaced tabs with proper padding
- print() takes two arguments, which are the GCNO and GCDA filenames
- Files are listed at the top of output, appended by line 0
- Stripped strings of trailing \0s
- Removed last two lines of whitespace in output

Patch by Yuchen Wu!