[ConstantFoldTerminator] Preserve make.implicit metadata when converting SwitchInst to BranchInst

Summary: llvm::ConstantFoldTerminator function can convert SwitchInst with single case (and default) to a conditional BranchInst. This patch adds support to preserve make.implicit metadata on this conversion.

Reviewers: sanjoy, weimingz, chenli

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D11841

llvm-svn: 244348