compiler: Avoid generating collapse(1)

[FabioLuporini]

Reason: NVC (at least version <= 23.1 , possible future as well) has a compilation bug (as in, the NVC compiler throws an internal fatal error while attempting to compile perfectly legal C+OpenMP code) that sometimes crops up when the collapse(1) clause is used.

Since (from the OpenMP specification)

The collapse clause associates one or more loops with the directive on which it appears for the purpose of identifying the portion of the depth of the canonical loop nest to which to apply the semantics of the directive. The argument n specifies the number of loops of the associated loop nest to which to apply those semantics. On all directives on which the collapse clause may appear, the effect is as if a value of one was specified for n if the collapse clause is not specified.

We here work around the annoying NVC issue by simply not generating collapse(1).

Bug hunting credits to @deckerla

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[codecov]

Codecov Report

Merging #2129 (58efbeb) into master (526dcb8) will increase coverage by 0.14%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #2129      +/-   ##
==========================================
+ Coverage   87.03%   87.17%   +0.14%     
==========================================
  Files         219      222       +3     
  Lines       38996    39383     +387     
  Branches     5105     5121      +16     
==========================================
+ Hits        33941    34334     +393     
+ Misses       4481     4478       -3     
+ Partials      574      571       -3     

Impacted Files	Coverage Δ
tests/test_gpu_openacc.py	5.69% <ø> (ø)
devito/passes/iet/languages/openacc.py	61.46% <66.66%> (ø)
tests/test_dle.py	96.47% <92.30%> (+0.02%)	⬆️
devito/passes/iet/languages/openmp.py	88.23% <100.00%> (+0.11%)	⬆️

... and 6 files with indirect coverage changes

[georgebisbas]

I do not see any problem with the changes, though there are failing tests