GC sometimes reports survival rates above 100%

[markples]

Easiest repro is to only allocate large objects - gcperfsim -tc 36 -tagb 100 -tlgb 0 -lohar 1000 -pohar 0 -sohsr 100-4000 -lohsr 16002400-16004800 -pohsr 100-204800 -sohsi 0 -lohsi 0 -pohsi 0 -sohpi 0 -lohpi 0 -sohfi 0 -lohfi 0 -pohfi 0 -allocType reference -testKind time - but this requires a separate gcperfsim fix.

This is due to allocations occurring during BGC. They are counted as survived but not at the start (i.e., in the numerator but not the denominator). It looks like inaccuracies are possible for all generations, but it easiest to expose it as >100% with large objects.

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[markples]

This issue is still relevant for ephemeral generations, but the impact there is closer to rounding error than the large swings that can be seen in LOH (due to the small -number- of allocations).