[JumpThreading] Change a dyn_cast that is already protected by an isa check to a static cast. Combine the with another static cast. NFC
[JumpThreading] Change a dyn_cast that is already protected by an isa check to a static cast. Combine the with another static cast. NFC

Differential Revision: https://reviews.llvm.org/D32874

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@302197 91177308-0d34-0410-b5e6-96231b3b80d8

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