Int**Int yields bogus overflow

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Migrated from rt.perl.org#130153 (status was 'new')

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[p6rt]

From zefram@fysh.org

2e0**-1019
1.78005908680576e-307
2e0**-1020
8.90029543402881e-308
2**-1019
1.78005908680576e-307
2**-1020
Numeric overflow
  in block <unit> at <unknown file> line 1

The result for 2**-1020 is bogus. The exact value of the operation
is representable in the Num type, as was used to represent the result
of 2**-1019.

-zefram

[p6rt]

From @zoffixznet

MoarVM PR fixing this sent​: MoarVM/MoarVM#442