compiler: Add optimization option to fuse WithLocks tasks

[FabioLuporini]

No description provided.

[codecov]

Codecov Report

Merging #1736 (5dad92c) into master (02bbefb) will decrease coverage by 0.40%.
The diff coverage is 27.38%.

❗ Current head 5dad92c differs from pull request most recent head f114164. Consider uploading reports for the commit f114164 to get more accurate results

@@            Coverage Diff             @@
##           master    #1736      +/-   ##
==========================================
- Coverage   89.63%   89.23%   -0.41%     
==========================================
  Files         209      204       -5     
  Lines       33664    33176     -488     
  Branches     4367     4342      -25     
==========================================
- Hits        30176    29603     -573     
- Misses       2991     3065      +74     
- Partials      497      508      +11     

Impacted Files	Coverage Δ
devito/ir/iet/nodes.py	92.12% <ø> (ø)
devito/passes/iet/orchestration.py	59.88% <0.00%> (-2.11%)	⬇️
tests/test_gpu_common.py	1.58% <0.00%> (-0.08%)	⬇️
devito/passes/iet/languages/openacc.py	93.47% <50.00%> (ø)
devito/passes/iet/parpragma.py	89.60% <50.00%> (-1.06%)	⬇️
devito/passes/clusters/misc.py	90.27% <62.50%> (-2.88%)	⬇️
devito/core/cpu.py	100.00% <100.00%> (ø)
devito/core/gpu.py	96.27% <100.00%> (+0.01%)	⬆️
devito/passes/iet/langbase.py	97.58% <100.00%> (+0.03%)	⬆️
examples/seismic/elastic/elastic_example.py	47.22% <0.00%> (-30.56%)	⬇️
... and 23 more

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[FabioLuporini]

@mloubout any objections?