compiler: Fix min/max reductions to be backend-portable

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (1428bbc) 86.69% compared to head (3e504f9) 86.69%.

Files	Patch %	Lines
devito/ir/clusters/algorithms.py	94.44%	0 Missing and 2 partials :warning:
devito/passes/iet/misc.py	77.77%	1 Missing and 1 partial :warning:
tests/test_dle.py	89.47%	1 Missing and 1 partial :warning:
devito/symbolics/extended_sympy.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2315 +/- ## ======================================= Coverage 86.69% 86.69% ======================================= Files 229 229 Lines 42983 43043 +60 Branches 7967 7983 +16 ======================================= + Hits 37262 37318 +56 - Misses 5033 5034 +1 - Partials 688 691 +3 ```

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