[MergeFunctions] Remove alias support.

The alias support was dead code since 2011. It was last touched
in r124182, where it was reintroduced after being removed
in r110434, and since then it was gated behind a HasGlobalAliases
flag that was permanently stuck as `false`.

It is also broken. I'm not sure if it bitrotted or was just broken
in the first place because it appears to have never been tested,
but the following IR results in a crash:

    define internal i32 @a(i32 %a, i32 %b) unnamed_addr {
      %c = add i32 %a, %b
      %d = xor i32 %a, %c
      ret i32 %c
    }

    define internal i32 @b(i32 %a, i32 %b) unnamed_addr {
      %c = add i32 %a, %b
      %d = xor i32 %a, %c
      ret i32 %c
    }

It seems safe to remove buggy untested code that no one cared about
for seven years.

Differential Revision: https://reviews.llvm.org/D34802

llvm-svn: 309313