[WebAssembly] Fix debug locations for ExplicitLocals pass (#2)

[aardappel]

Summary

This is a reworked version of the reverted patch: https://reviews.llvm.org/D112487
Note that
a) it doesn't need the test changes anymore, and
b) I checked at least locally it passes other.test_pthread_lsan_leak

[dschuff]

LGTM. Let's get this in to fix https://bugs.chromium.org/p/chromium/issues/detail?id=1251909 and then we can investigate the Binaryen bug (and consider whether this or another approach is actually the best for these stackified local.gets) separately.

llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

for (MachineInstr &MI : llvm::make_early_inc_range(MBB)) {
           continue;
         }
 
         // Insert a local.get.
         unsigned LocalId = getLocalId(Reg2Local, MFI, CurLocal, OldReg);
         const TargetRegisterClass *RC = MRI.getRegClass(OldReg);
         Register NewReg = MRI.createVirtualRegister(RC);
         unsigned Opc = getLocalGetOpcode(RC);
-        InsertPt =
-            BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc), NewReg)
-                .addImm(LocalId);
+        // Use a InsertPt as our DebugLoc, since MI may be discontinuous from
+        // the where this local is being inserted, causing non-linear stepping
+        // in the debugger or function entry points where variables aren't live
+        // yet. Alternative is previous instruction, but that is strictly worse
+        // since it can point at the previous statement.
+        // See crbug.com/1251909, crbug.com/1249745
+        InsertPt = BuildMI(MBB, InsertPt, InsertPt->getDebugLoc(),
+                           TII->get(Opc), NewReg).addImm(LocalId);
         MO.setReg(NewReg);
         MFI.stackifyVReg(MRI, NewReg);
         Changed = true;
       }
 
       // Coalesce and eliminate COPY instructions.
       if (WebAssembly::isCopy(MI.getOpcode())) {
         MRI.replaceRegWith(MI.getOperand(1).getReg(),