[SDAG] Use useDebugInstrRef instead of shouldUseDebugInstrRef (#160686)

`shouldUseDebugInstrRef` can return different value than
`useDebugInstrRef`, since the first depends on opt level which can
change. Inconsistent usage can lead to errors later.

I believe that using `should...` instead of `use...` here is a result of
a minor error during this:
https://github.com/llvm/llvm-project/pull/94149/files#diff-8ec547e1244562c5837ed180dd9bed61b3cd960ef90bb6002ea2db41a67ed693

Notice how before the change `InstrRef` is assigned value from
`should...` *before* the opt change. Now, it's done after -- opt change
happens here:
```c
bool SelectionDAGISelLegacy::runOnMachineFunction(MachineFunction &MF) {
...
  // Decide what flavour of variable location debug-info will be used, before
  // we change the optimisation level.
  MF.setUseDebugInstrRef(MF.shouldUseDebugInstrRef());
....

  return Selector->runOnMachineFunction(MF);
}
```

Then `runOnMachineFunction` uses `should...`, which after opt change may
return different value than it did previously.

Author: mikolaj-pirog

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -571,7 +571,7 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
   SwiftError->setFunction(mf);
   const Function &Fn = mf.getFunction();
 
-  bool InstrRef = mf.shouldUseDebugInstrRef();
+  bool InstrRef = mf.useDebugInstrRef();
 
   FuncInfo->set(MF->getFunction(), *MF, CurDAG);
 

llvm/test/DebugInfo/X86/instr-ref-opt-bisect2.ll

@@ -0,0 +1,36 @@
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=1 | FileCheck %s
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=10 | FileCheck %s
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=100 | FileCheck %s
+
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=1 -fast-isel=true | FileCheck %s
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=10 -fast-isel=true | FileCheck %s
+; RUN: llc %s -o - -stop-after=livedebugvalues -opt-bisect-limit=100 -fast-isel=true | FileCheck %s
+
+; This test has the same purpose as the instr-ref-opt-bisect.ll, to check if
+; during opt-bisect's optimisation level change we won't run into an assert.
+; This is simply testing different IR.
+
+; CHECK: DBG_VALUE
+
+target triple = "x86_64-pc-windows-msvc"
+
+define i1 @foo(i32 %arg) !dbg !3 {
+entry:
+    #dbg_value(i32 %arg, !4, !DIExpression(), !5)
+  switch i32 %arg, label %bb [
+    i32 810, label %bb
+  ], !dbg !5
+bb:
+  %a = load volatile i1, ptr null, align 1
+  ret i1 false
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1)
+!1 = !DIFile(filename: "instr-ref-opt-bisect2.ll", directory: ".")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = distinct !DISubprogram(name: "instr-ref-opt-bisect2", file: !1, unit: !0)
+!4 = !DILocalVariable(name: "arg", arg: 2, scope: !3)
+!5 = !DILocation(line: 0, scope: !3)