[Utils] Salvage the debug info of DCE'ed 'xor' instructions

This salvages 259 debug values in a stage2 build of clang.

Differential Revision: https://reviews.llvm.org/D43207

llvm-svn: 324973
diff --git a/llvm/test/Transforms/InstCombine/debuginfo-variables.ll b/llvm/test/Transforms/InstCombine/debuginfo-variables.ll
index 39a270f..d2cd7ea 100644
--- a/llvm/test/Transforms/InstCombine/debuginfo-variables.ll
+++ b/llvm/test/Transforms/InstCombine/debuginfo-variables.ll
@@ -17,6 +17,13 @@
   ret void
 }
 
+define void @test_xor(i32 %A) {
+; CHECK-LABEL: @test_xor(
+; CHECK-NEXT:  call void @llvm.dbg.value(metadata i32 %A, metadata !22, metadata !DIExpression(DW_OP_constu, 1, DW_OP_xor, DW_OP_stack_value)), !dbg !23
+  %1 = xor i32 %A, 1
+  ret void
+}
+
 ; CHECK: !8 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !9)
 ; CHECK: !10 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 2, type: !11)
 ; CHECK: !12 = !DILocation(line: 2, column: 1, scope: !5)
@@ -24,3 +31,6 @@
 
 ; CHECK: !17 = !DILocalVariable(name: "3", scope: !15, file: !1, line: 4, type: !11)
 ; CHECK: !18 = !DILocation(line: 4, column: 1, scope: !15)
+
+; CHECK: !22 = !DILocalVariable(name: "4", scope: !20, file: !1, line: 6, type: !9)
+; CHECK: !23 = !DILocation(line: 6, column: 1, scope: !20)