Simplified get_blkno_indexers

[WillAyd]

This function is the last that appears to be throwing warnings in #32163

This PR doesn't solve that, but simplifies the logic to make it easier to grok

[jbrockmendel]

nice cleanup assuming perf isnt hurt

[WillAyd]

Here are the results for reshaping and concat. A mixed bag but I think net benefit

+     1.50±0.08ms       2.80±0.2ms     1.87  reshape.SparseIndex.time_unstack
-        30.6±3ms         18.9±2ms     0.62  reshape.Cut.time_qcut_datetime(10)
-        89.8±5ms         50.9±6ms     0.57  reshape.Cut.time_qcut_timedelta(1000)
-         101±7ms         48.7±5ms     0.48  reshape.Cut.time_qcut_datetime(1000)
-        31.9±3ms         14.8±1ms     0.46  reshape.Cut.time_qcut_datetime(4)
-       94.1±10ms         36.6±8ms     0.39  reshape.Cut.time_cut_datetime(1000)
-        19.7±5ms       7.51±0.2ms     0.38  reshape.Cut.time_cut_datetime(4)

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE DECREASED.

       before           after         ratio
     [1f2385ea]       [dec831fa]
+        18.4±1ms         26.5±2ms     1.44  join_merge.Concat.time_concat_small_frames(1)
-        843±80μs          649±9μs     0.77  join_merge.Concat.time_concat_mixed_ndims(1)
-         327±7μs          245±3μs     0.75  join_merge.Concat.time_concat_empty_right(1)
-         373±5μs         254±10μs     0.68  join_merge.Concat.time_concat_mixed_ndims(0)
-        377±30μs          251±3μs     0.67  join_merge.Concat.time_concat_empty_left(1)
-        286±70μs          182±4μs     0.64  join_merge.Concat.time_concat_empty_left(0)

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE DECREASED.

[jreback]

thanks