[SelectionDAG] Early-out in TargetLowering::expandMUL (NFC)
[SelectionDAG] Early-out in TargetLowering::expandMUL (NFC)

Summary: Reduce indentation level; preparation for D24956.

Reviewers: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27063

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@287831 91177308-0d34-0410-b5e6-96231b3b80d8

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