runtime: add explicit lower bounds check to decoderune

[uellenberg]

By doing a full bounds check here, we eliminate the auto generated one,
which makes the codegen slightly better.

Requires I39ba2a18cbabc0559924d4d226dcb99dbe9a06ed for BCE, although
this can be merged before it.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Keith Randall:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/725101.
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