Hadrian: fix ghcDebugAssertions off-by-one error

Commit 6b2f7ffe changed the logic that decided whether to enable debug
assertions. However, it had an off-by-one error, as the stage parameter
to the function inconsistently referred to the stage of the compiler
being used to build or the stage of the compiler we are building.

This patch makes it consistent. Now the parameter always refers to the
the compiler which is being built.

In particular, this patch re-enables
assertions in the stage 2 compiler when building with devel2 flavour,
and disables assertions in the stage 2 compiler when building with
validate flavour.

Some extra performance tests are now run in the "validate" jobs because
the stage2 compiler no longer contains assertions.

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Metric Decrease:
    CoOpt_Singletons
    MultiComponentModules
    MultiComponentModulesRecomp
    MultiLayerModulesTH_OneShot
    T11374
    T12227
    T12234
    T13253-spj
    T13701
    T14683
    T14697
    T15703
    T17096
    T17516
    T18304
    T18478
    T18923
    T5030
    T9872b
    TcPlugin_RewritePerf
Metric Increase:
    MultiComponentModules
    MultiComponentModulesRecomp
    MultiLayerModules
    MultiLayerModulesRecomp
    MultiLayerModulesTH_Make
    T13386
    T13719
    T3294
    T9233
    T9675
    parsing001
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