[llvm] [SelectionDAG] Salvage debuginfo when combining load and z|s ext instrs. (PR #188544)

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -0,0 +1,48 @@
+; This test checks that after SelectionDAG runs, it preserves the debug info that is lost due to the DAGCombiner combining a load and a zext instruction, where the #dbg_value is pointing to the result of the load. However, this test also ensures that the DIExpression, which has multiple DW_OP_LLVM_arg's is handled correctly when the debug info is preserved in selectioDAG.
+; RUN: llc %s -mtriple=x86_64-unkown-linux-gnu -start-before=x86-isel -stop-after=x86-isel -o - | FileCheck %s --check-prefix=MIR
+; RUN: llc -O2 %s -start-before=x86-isel -mtriple=x86_64-unkown-linux --filetype=obj -o %t.o 
+
+; MIR: ![[V:[0-9]+]] = !DILocalVariable(name: "v"
+; MIR-LABEL: bb.0
+; MIR: %{{[0-9a-f]+}}{{.*}} = MOVZX32rm8 {{.*}}, 1, $noreg, 0, $noreg, debug-instr-number [[INSTR_NUM:[0-9]+]]
+; MIR-NEXT: DBG_INSTR_REF ![[V]], !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_constu, 45, DW_OP_eq, DW_OP_LLVM_arg, 1, DW_OP_constu, 114, DW_OP_eq, DW_OP_or, DW_OP_LLVM_convert, 8, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value), dbg-instr-ref([[INSTR_NUM]], 0), dbg-instr-ref([[INSTR_NUM]], 0)
----------------
OCHyams wrote:

I'm wondering whether the conversion needs to happen after each `DW_OP_LLVM_arg` use rather than at the end of the entire expression?

https://github.com/llvm/llvm-project/pull/188544