Instead of crashing when MCAsmInfo is NULL, add an assert.

This helps with porting code from 2.9 to 3.0 as TargetSelect.h changed location,
and if you include the old one by accident you will trigger this assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140848 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/LLVMTargetMachine.cpp b/lib/CodeGen/LLVMTargetMachine.cpp
index 8f463e2..3bd1d60 100644
--- a/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/lib/CodeGen/LLVMTargetMachine.cpp
@@ -114,6 +114,11 @@
   : TargetMachine(T, Triple, CPU, FS) {
   CodeGenInfo = T.createMCCodeGenInfo(Triple, RM, CM);
   AsmInfo = T.createMCAsmInfo(Triple);
+  // TargetSelect.h moved to different directory between LLVM 2.9 and 3.0,
+  // and if the old one gets included then MCAsmInfo will be NULL and we'd crash
+  // later.
+  // Provide the user a useful error message about whats wrong.
+  assert(AsmInfo && "MCAsmInfo not initialized. Make sure you include the correct TargetSelect.h!");
 }
 
 bool LLVMTargetMachine::addPassesToEmitFile(PassManagerBase &PM,