[Alignment] fix dubious min function alignment

Summary:
This was discovered while introducing the llvm::Align type.
The original setMinFunctionAlignment used to take alignment as log2, looking at the comment it seems like instructions are to be 2-bytes aligned and not 4-bytes aligned.

Reviewers: uweigand

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67271

llvm-svn: 371204