[llvm-locstats] Fix labels on x-axis of comparison chart

[vsk]

Summary

This change makes the labels on the x-axis of a comparison chart look
like: "0%", "(0%, 10%)", "[10%, 20%)", and so on.

Previously, each label was the same (a concatenation of all the possible
coverage buckets).

[djtodoro]

LGTM. Thanks!

llvm/utils/llvm-locstats/llvm-locstats.py

def draw_location_diff(self, locstats_to_compare):
 
     buckets = range(len(self.variables_coverage_map))
     buckets_to_compare = range(len(locstats_to_compare.variables_coverage_map))
 
     fig = plt.figure(figsize=(12, 8))
     ax = fig.add_subplot(111)
     init_plot(plt)
 
+    comparison_keys = list(coverage_buckets())
     ax.bar(buckets, self.variables_coverage_map.values(), align='edge',
-           tick_label=self.variables_coverage_map.keys(), width=0.4,
+           width=0.4,
            label='variables of {}'.format(self.file_name))
     ax.bar(buckets_to_compare,
            locstats_to_compare.variables_coverage_map.values(),
            color='r', align='edge', width=-0.4,
-           tick_label=locstats_to_compare.variables_coverage_map.keys(),
            label='variables of {}'.format(locstats_to_compare.file_name))
+    ax.set_xticks(range(len(comparison_keys)))
+    ax.set_xticklabels(comparison_keys)
 
     props = dict(boxstyle='round', facecolor='wheat', alpha=0.5)
     plt.text(0.02, 0.88,
              '{} PC ranges covered: {}%'. \
              format(self.file_name, pc_ranges_covered),
              transform=plt.gca().transAxes, fontsize=12,
              verticalalignment='top', bbox=props)
     plt.text(0.02, 0.83,