GRHydro: work around Intel 19 compiler issue

[Roland Haas]

The intel 19 compiler and newer apparently miscompile GRhydro’s HLLE code when MHD is enabled. This results in either a SEGFAULT of con2prim failures. This has been in the ET tests since around 2021 (it shows up in the ET_2021_05_v0 release tests).

Pull request
https://bitbucket.org/einsteintoolkit/einsteinevolve/pull-requests/18
provides a workaround. With it the affected tests pass, see eg qb an supermuc on http://einsteintoolkit.org/testsuite_results/index.php

[Roland Haas]

Please review.

[Roland Haas]

@Erik Schnetter says (private conversation): please apply.

[Roland Haas]

Applied as git hash 33a5f20 "GRHydro: work around Intel 19 compiler issue" of einsteinevolve