[MergeFunc] Allow merging identical vararg functions using aliases

Thanks to Nikita Popov for pointing out this missed case.

This is a follow-up to r351411, which disabled function merging for
vararg functions outright due to a miscompile (see llvm.org/PR40345).

Differential Revision: https://reviews.llvm.org/D56865

llvm-svn: 351624