Fix variable name.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183752 91177308-0d34-0410-b5e6-96231b3b80d8
Port r183666 to identify_magic.

It will be tested in the next commit which moves another user to identify_magic.

Original message:

Fix an out of bounds array access.

We were looking at Magic[5] without checking Length. Since this path would not
return unless Length >= 18 anyway, just move the >= 18 check up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183753 91177308-0d34-0410-b5e6-96231b3b80d8
Convert another use of sys::identifyFileType.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183754 91177308-0d34-0410-b5e6-96231b3b80d8
Change how globalopt handles aliases in llvm.used.

Instead of a custom implementation of replaceAllUsesWith, we just call
replaceAllUsesWith and recreate llvm.used and llvm.compiler-used.

This change is particularity interesting because it makes llvm see
through what clang is doing with static used functions in extern "C"
contexts. With this change, running clang -O2 in

extern "C" {
  __attribute__((used)) static void foo() {}
}

produces

@llvm.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to
i8*)], section "llvm.metadata"
define internal void @foo() #0 {
entry:
  ret void
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183756 91177308-0d34-0410-b5e6-96231b3b80d8