llvm-cov: Use uint32_t for loop variables to be more consistent.

The loop bounds here are uint32_t variables, so it makes sense for the
loop variables to have the same type.

Patch by Yuchen Wu!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193192 91177308-0d34-0410-b5e6-96231b3b80d8