Add hook methods to analysis.Frame for branch-sensitive analyses

Adds two hook methods to `analysis.Frame`, called by `Analyzer.analyze`.
These methods are called on the temporary `currentFrame` before it is
merged with the existing frame at the branch target.

Overriding these methods allows implementing branch-sensitive analyses,
for example for nullness.
1 job for branch-sensitive in 8 minutes and 36 seconds (queued for 3 seconds)
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Status Job ID Name Coverage
  Test
passed #7574
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00:08:36