[DebugInfo][RemoveDIs] Use splice in Outliner rather than moveBefore (#79124)

This patch replaces a utility in the outliner that moves the contents of
one basic block into another basic block, with a call to splice instead.
I think it's NFC, however I'd like a second pair of eyes to look at it
just in case.

The reason for doing this is an edge case in the handling of DPValue
objects, the replacement for dbg.values. If there's a variable
assignment "dangling" at the end of a block (which happens when we
delete the terminator), inserting instructions at end() doesn't shift
the DPValue up into the block. We could probably fix this; but it's much
easier to use splice at the only call site that does this.

Patch adds --try-experimental-debuginfo-iterators to a test to exercise
this code path.

Author: jmorse

llvm/lib/Transforms/IPO/IROutliner.cpp

@@ -154,8 +154,7 @@ struct OutlinableGroup {
 /// \param SourceBB - the BasicBlock to pull Instructions from.
 /// \param TargetBB - the BasicBlock to put Instruction into.
 static void moveBBContents(BasicBlock &SourceBB, BasicBlock &TargetBB) {
-  for (Instruction &I : llvm::make_early_inc_range(SourceBB))
-    I.moveBeforePreserving(TargetBB, TargetBB.end());
+  TargetBB.splice(TargetBB.end(), &SourceBB);
 }
 
 /// A function to sort the keys of \p Map, which must be a mapping of constant

llvm/test/Transforms/IROutliner/legal-debug.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --include-generated-funcs
 ; RUN: opt -S -passes=verify,iroutliner -ir-outlining-no-cost < %s | FileCheck %s
+; RUN: opt -S -passes=verify,iroutliner -ir-outlining-no-cost < %s --try-experimental-debuginfo-iterators | FileCheck %s
 
 ; This test checks that debug info is recognized as able to be extracted along
 ; with the other instructions, but is not included in the consolidated function.