[DebugInfo][Dexter] Speculated BB presents illegal variable value to debugger. 

When SimplifyCFG changes the PHI node into a select instruction, the debug information becomes ambiguous. It causes the debugger to display wrong variable value. 

Differential Revision: https://reviews.llvm.org/D51976

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342527 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed