[ConstantFolding] Support folding loads from a GlobalAlias

The MSVC ABI requires that we generate an alias for the vtable which
means looking through a GlobalAlias which cannot be overridden improves
our ability to devirtualize.

Found while investigating PR20801.

Patch by Andrew Zhogin!

Differential Revision: http://reviews.llvm.org/D11306

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242955 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SCCP/global-alias-constprop.ll

@@ -0,0 +1,11 @@
+; RUN: opt < %s -sccp -S | FileCheck %s
+
+@0 = private unnamed_addr constant [2 x i32] [i32 -1, i32 1]
+@"\01??_7A@@6B@" = unnamed_addr alias getelementptr inbounds ([2 x i32], [2 x i32]* @0, i32 0, i32 1)
+
+; CHECK: ret i32 1
+
+define i32 @main() {
+  %a = load i32, i32* @"\01??_7A@@6B@"
+  ret i32 %a
+}