compiler: Improve lowering of IndexDerivatives

[FabioLuporini]

More use cases covered, better generated code (e.g. more fusion).

PR equipped with tests

[codecov]

Codecov Report

Merging #2112 (8077f6e) into master (51e11e5) will increase coverage by 0.04%.
The diff coverage is 98.15%.

@@            Coverage Diff             @@
##           master    #2112      +/-   ##
==========================================
+ Coverage   87.09%   87.14%   +0.04%     
==========================================
  Files         222      223       +1     
  Lines       39523    39647     +124     
  Branches     5127     5139      +12     
==========================================
+ Hits        34424    34550     +126     
  Misses       4521     4521              
+ Partials      578      576       -2     

Impacted Files	Coverage Δ
devito/tools/abc.py	88.57% <50.00%> (-1.63%)	⬇️
devito/ir/support/basic.py	92.86% <87.50%> (-0.11%)	⬇️
devito/passes/clusters/misc.py	91.06% <89.47%> (+1.19%)	⬆️
devito/types/misc.py	96.20% <91.66%> (-1.10%)	⬇️
devito/ir/clusters/algorithms.py	95.31% <100.00%> (-0.08%)	⬇️
devito/operator/operator.py	89.19% <100.00%> (+0.02%)	⬆️
devito/passes/clusters/derivatives.py	99.01% <100.00%> (+0.35%)	⬆️
devito/passes/iet/linearization.py	82.93% <100.00%> (ø)
devito/passes/iet/misc.py	88.88% <100.00%> (+2.81%)	⬆️
devito/types/dimension.py	94.31% <100.00%> (+0.57%)	⬆️
... and 4 more

[georgebisbas]

I think it looks good to my eyes

[FabioLuporini]

Merged