[mips] Use isa<> instead of dyn_cast<> with unused value. NFC.
[mips] Use isa<> instead of dyn_cast<> with unused value. NFC.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7615

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@229138 91177308-0d34-0410-b5e6-96231b3b80d8

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