[Mips] Add the missing judgment when processing function handleMFLOSlot

In function handleMFLOSlot, we may get a variable LastInstInFunction
with a value of true from function getNextMachineInstr and IInSlot may
be null which would trigger an assert.
So we need to skip this case.

Fix #118223.

Author: yingopq

llvm/lib/Target/Mips/MipsBranchExpansion.cpp

@@ -767,6 +767,8 @@ bool MipsBranchExpansion::handleMFLOSlot(Pred Predicate, Safe SafeInSlot) {
         std::pair<Iter, bool> Res = getNextMachineInstr(std::next(I), &*FI);
         LastInstInFunction |= Res.second;
         IInSlot = Res.first;
+        if (LastInstInFunction)
+          continue;
         if (!SafeInSlot(*IInSlot, *I)) {
           Changed = true;
           TII->insertNop(*(I->getParent()), std::next(I), I->getDebugLoc())

llvm/test/CodeGen/Mips/llvm-ir/sdiv-freebsd.ll

@@ -0,0 +1,67 @@
+; RUN: llc -filetype=asm < %s -mcpu=mips2 | FileCheck %s -check-prefixes=MIPS2
+;
+; Created from the following test case (PR121463) with
+; clang -cc1 -triple mips-unknown-freebsd -target-cpu mips2 -O2 -emit-llvm test.c -o test.ll
+; int l2arc_feed_secs, l2arc_feed_min_ms, l2arc_write_interval_wrote, l2arc_write_interval_next;
+; void l2arc_write_interval() {
+;   int interval;
+;   if (l2arc_write_interval_wrote)
+;     interval = l2arc_feed_min_ms / l2arc_feed_secs;
+;   l2arc_write_interval_next = interval;
+; }
+
+target datalayout = "E-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64"
+target triple = "mips-unknown-freebsd"
+
+@l2arc_write_interval_wrote = local_unnamed_addr global i32 0, align 4
+@l2arc_feed_min_ms = local_unnamed_addr global i32 0, align 4
+@l2arc_feed_secs = local_unnamed_addr global i32 0, align 4
+@l2arc_write_interval_next = local_unnamed_addr global i32 0, align 4
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(readwrite, argmem: none, inaccessiblemem: none)
+define dso_local void @l2arc_write_interval() local_unnamed_addr #0 {
+; MIPS2-LABEL: l2arc_write_interval:
+; MIPS2:       # %bb.0: # %entry
+; MIPS2-NEXT:    lui $1, %hi(l2arc_write_interval_wrote)
+; MIPS2-NEXT:    lw $1, %lo(l2arc_write_interval_wrote)($1)
+; MIPS2-NEXT:    beqz $1, $BB0_2
+; MIPS2-NEXT:    nop
+; MIPS2-NEXT:  # %bb.1: # %if.then
+; MIPS2-NEXT:    lui $1, %hi(l2arc_feed_secs)
+; MIPS2-NEXT:    lw $1, %lo(l2arc_feed_secs)($1)
+; MIPS2-NEXT:    lui $2, %hi(l2arc_feed_min_ms)
+; MIPS2-NEXT:    lw $2, %lo(l2arc_feed_min_ms)($2)
+; MIPS2-NEXT:    div $zero, $2, $1
+; MIPS2-NEXT:    teq $1, $zero, 7
+; MIPS2-NEXT:    mflo $2
+; MIPS2-NEXT:    j $BB0_3
+; MIPS2-NEXT:    nop
+entry:
+  %0 = load i32, ptr @l2arc_write_interval_wrote, align 4, !tbaa !2
+  %tobool.not = icmp eq i32 %0, 0
+  br i1 %tobool.not, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  %1 = load i32, ptr @l2arc_feed_min_ms, align 4, !tbaa !2
+  %2 = load i32, ptr @l2arc_feed_secs, align 4, !tbaa !2
+  %div = sdiv i32 %1, %2
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  %interval.0 = phi i32 [ %div, %if.then ], [ undef, %entry ]
+  store i32 %interval.0, ptr @l2arc_write_interval_next, align 4, !tbaa !2
+  ret void
+}
+
+attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(readwrite, argmem: none, inaccessiblemem: none) "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="mips2" "target-features"="+mips2" }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 20.0.0git ([email protected]:yingopq/llvm-project.git c23f2417dc5f6dc371afb07af5627ec2a9d373a0)"}
+!2 = !{!3, !3, i64 0}
+!3 = !{!"int", !4, i64 0}
+!4 = !{!"omnipotent char", !5, i64 0}
+!5 = !{!"Simple C/C++ TBAA"}
+