[GlobalISel] Update DBG_VALUEs referencing DCE'd instructions.

Quentin points out that r298358 would cause us to emit different code
with debug info.  That's a big no-no; also erase the instructions that
only live thanks to DBG_VALUE users.

Adrian explained how this is an existing problem and an OK thing to do:
clang has allocas for all variables so shouldn't be affected at -O0, but
swift uses a bit of inlineasm to explicitly keep values live for the
purpose of debug info quality.  I'm not sure there is a better scheme.

llvm-svn: 298460

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp b/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
index 69ad59ade08..26454c1ef00 100644
--- a/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/InstructionSelect.cpp
@@ -123,7 +123,7 @@ bool InstructionSelect::runOnMachineFunction(MachineFunction &MF) {
       // If so, erase it.
       if (isTriviallyDead(MI, MRI)) {
         DEBUG(dbgs() << "Is dead; erasing.\n");
-        MI.eraseFromParent();
+        MI.eraseFromParentAndMarkDBGValuesForRemoval();
         continue;
       }