compiler: prevent adding breaking guard to nokey

[mloubout]

No description provided.

[codecov]

Codecov Report

Merging #2160 (2eceda4) into master (474b10c) will decrease coverage by 0.02%.
The diff coverage is 64.70%.

@@            Coverage Diff             @@
##           master    #2160      +/-   ##
==========================================
- Coverage   87.07%   87.06%   -0.02%     
==========================================
  Files         223      223              
  Lines       39916    39931      +15     
  Branches     5173     5174       +1     
==========================================
+ Hits        34758    34765       +7     
- Misses       4580     4589       +9     
+ Partials      578      577       -1     

Impacted Files	Coverage Δ
devito/passes/clusters/buffering.py	91.18% <50.00%> (-0.98%)	⬇️
devito/ir/support/guards.py	59.50% <77.77%> (-3.89%)	⬇️

... and 2 files with indirect coverage changes