Fix harmless warning in dwarf_cfi_to_module.cc

Fix harmless warning in dwarf_cfi_to_module.cc.

This function already establishes that (signed) i must be positive by the time
it assigns it to (unsigned) reg. Because reg is unsigned, it is impossible for
it to be negative. Because i was already checked for positivity, this check
can be removed entirely. It was not a miswritten check that intended to check
i instead of reg, because i is already checked.

This addresses the following warning:

dwarf_cfi_to_module.cc:135:9: warning: comparison of 0 <= unsigned expression is
always true [-Wtautological-compare]

Committed r861

[qsr]

LGTM