[Windows SEH] Fix crash on empty seh block (llvm#107031)

Fixes llvm#105813 and
llvm#106915.
Adds a check for the end of the iterator, which can be a sentinel. 
The issue was introduced in
llvm@0efe111
from what I can see, so along with the introduction of /EHa support.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -1476,6 +1476,10 @@ void SelectionDAGISel::reportIPToStateForBlocks(MachineFunction *MF) {
     if (BB->getFirstMayFaultInst()) {
       // Report IP range only for blocks with Faulty inst
       auto MBBb = MBB.getFirstNonPHI();
+
+      if (MBBb == MBB.end())
+        continue;
+
       MachineInstr *MIb = &*MBBb;
       if (MIb->isTerminator())
         continue;

llvm/test/CodeGen/WinEH/wineh-empty-seh-scope.ll

@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=x86_64-pc-windows-msvc19.41.34120 < %s | FileCheck %s
+
+define void @foo() personality ptr @__CxxFrameHandler3 {
+; CHECK-LABEL: foo:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    nop # avoids zero-length function
+  call void @llvm.seh.scope.begin()
+  unreachable
+}
+
+declare i32 @__CxxFrameHandler3(...)
+
+declare void @llvm.seh.scope.begin()
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"eh-asynch", i32 1}