Fix a bug when unswitching on partial LIV for SwitchInst

Summary: Fix a bug when unswitching on partial LIV for SwitchInst.

Reviewers: hfinkel, efriedma, sanjoy

Reviewed By: sanjoy

Subscribers: david2050, mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D29107

llvm-svn: 296363