[CodeGen][X86] LiveRangeShrink: fix increment after end (#115276)

This fixes the infinite loop discovered in #114195. 
Since we skip debug instructions at the start of the loop we do not need
to skip them again at the end of the loop.

Author: MuellerMP

llvm/lib/CodeGen/LiveRangeShrink.cpp

@@ -246,7 +246,7 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
         if (MI.getOperand(0).isReg())
           for (; EndIter != MBB.end() && EndIter->isDebugValue() &&
                  EndIter->hasDebugOperandForReg(MI.getOperand(0).getReg());
-               ++EndIter, ++Next)
+               ++EndIter)
             IOM[&*EndIter] = NewOrder;
         MBB.splice(I, &MBB, MI.getIterator(), EndIter);
       }

llvm/test/CodeGen/X86/lrshrink-debug.ll

@@ -0,0 +1,82 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc %s -o - | FileCheck %s
+target triple = "i686-unknown-linux-gnu"
+
+define noundef i32 @test(i1 %tobool1.not, i32 %sh.012, i1 %cmp, i64 %sh_prom, i64 %shl) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    pushl %ebp
+; CHECK-NEXT:    .cfi_def_cfa_offset 8
+; CHECK-NEXT:    pushl %ebx
+; CHECK-NEXT:    .cfi_def_cfa_offset 12
+; CHECK-NEXT:    pushl %edi
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    pushl %esi
+; CHECK-NEXT:    .cfi_def_cfa_offset 20
+; CHECK-NEXT:    .cfi_offset %esi, -20
+; CHECK-NEXT:    .cfi_offset %edi, -16
+; CHECK-NEXT:    .cfi_offset %ebx, -12
+; CHECK-NEXT:    .cfi_offset %ebp, -8
+; CHECK-NEXT:    xorl %esi, %esi
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; CHECK-NEXT:    movzbl {{[0-9]+}}(%esp), %edx
+; CHECK-NEXT:    movb {{[0-9]+}}(%esp), %dh
+; CHECK-NEXT:    xorl %edi, %edi
+; CHECK-NEXT:    jmp .LBB0_1
+; CHECK-NEXT:    .p2align 4
+; CHECK-NEXT:  .LBB0_4: # %if.end
+; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    orl %ecx, %ebx
+; CHECK-NEXT:    orl %eax, %ebp
+; CHECK-NEXT:    movl %ebx, %esi
+; CHECK-NEXT:    movl %ebp, %edi
+; CHECK-NEXT:  .LBB0_1: # %for.body
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    testb $1, %dh
+; CHECK-NEXT:    je .LBB0_1
+; CHECK-NEXT:  # %bb.2: # %if.end
+; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    xorl %ebx, %ebx
+; CHECK-NEXT:    testb $1, %dl
+; CHECK-NEXT:    movl $0, %ebp
+; CHECK-NEXT:    jne .LBB0_4
+; CHECK-NEXT:  # %bb.3: # %if.end
+; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    movl %esi, %ebx
+; CHECK-NEXT:    movl %edi, %ebp
+; CHECK-NEXT:    jmp .LBB0_4
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.inc, %entry
+  %bitmap.013 = phi i64 [ 0, %entry ], [ %bitmap.2, %for.inc ]
+  br i1 %tobool1.not, label %if.end, label %for.inc
+
+if.end:                                           ; preds = %for.body
+  %spec.select10 = select i1 %cmp, i64 0, i64 %bitmap.013
+  %shl6 = shl nuw i64 1, %sh_prom
+  %or = or i64 %shl, %spec.select10
+  tail call void @llvm.dbg.value(metadata i64 %or, metadata !17, metadata !DIExpression()), !dbg !21
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end, %for.body
+  %bitmap.2 = phi i64 [ %bitmap.013, %for.body ], [ %or, %if.end ]
+  %tobool.not = icmp eq i32 0, 0
+  br label %for.body
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!16}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, enums: !2)
+!1 = !DIFile(filename: "test.c", directory: "test")
+!2 = !{}
+!16 = !{i32 2, !"Debug Info Version", i32 3}
+!17 = !DILocalVariable(name: "bitmap", scope: !18, file: !1, line: 8, type: !20)
+!18 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 6, type: !19, scopeLine: 6, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!19 = !DISubroutineType(types: !2)
+!20 = !DIBasicType(name: "long long", size: 64, encoding: DW_ATE_signed)
+!21 = !DILocation(line: 0, scope: !18)