[CodeGen] Add an -O0 backend pipeline test. NFC.

This should hopefully makes changes to the O0 pipeline obvious; it's
easy to require expensive passes, and this helps make informed
decisions.

Case in point: in the few weeks separating the time when I initially
wrote this patch to the time when I committed, the test regressed as
r302103 added another use of DT!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302608 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/X86/O0-pipeline.ll

@@ -0,0 +1,71 @@
+; RUN: llc -mtriple=x86_64-- -O0 -debug-pass=Structure < %s -o /dev/null 2>&1 | FileCheck %s
+
+; REQUIRES: asserts
+
+; CHECK-LABEL: Pass Arguments:
+; CHECK-NEXT: Target Library Information
+; CHECK-NEXT: Target Transform Information
+; CHECK-NEXT: Target Pass Configuration
+; CHECK-NEXT: Type-Based Alias Analysis
+; CHECK-NEXT: Scoped NoAlias Alias Analysis
+; CHECK-NEXT: Assumption Cache Tracker
+; CHECK-NEXT: Create Garbage Collector Module Metadata
+; CHECK-NEXT: Machine Module Information
+; CHECK-NEXT: Machine Branch Probability Analysis
+; CHECK-NEXT:   ModulePass Manager
+; CHECK-NEXT:     Pre-ISel Intrinsic Lowering
+; CHECK-NEXT:     FunctionPass Manager
+; CHECK-NEXT:       Expand Atomic instructions
+; CHECK-NEXT:       Dominator Tree Construction
+; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:       Module Verifier
+; CHECK-NEXT:       Lower Garbage Collection Instructions
+; CHECK-NEXT:       Shadow Stack GC Lowering
+; CHECK-NEXT:       Remove unreachable blocks from the CFG
+; CHECK-NEXT:       Inserts calls to mcount-like functions
+; CHECK-NEXT:     Rewrite Symbols
+; CHECK-NEXT:     FunctionPass Manager
+; CHECK-NEXT:       Dominator Tree Construction
+; CHECK-NEXT:       Exception handling preparation
+; CHECK-NEXT:       Dominator Tree Construction
+; CHECK-NEXT:       Natural Loop Information
+; CHECK-NEXT:       Scalar Evolution Analysis
+; CHECK-NEXT:       Safe Stack instrumentation pass
+; CHECK-NEXT:       Insert stack protectors
+; CHECK-NEXT:       Module Verifier
+; CHECK-NEXT:       Dominator Tree Construction
+; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:       Function Alias Analysis Results
+; CHECK-NEXT:       X86 DAG->DAG Instruction Selection
+; CHECK-NEXT:       X86 PIC Global Base Reg Initialization
+; CHECK-NEXT:       Expand ISel Pseudo-instructions
+; CHECK-NEXT:       Local Stack Slot Allocation
+; CHECK-NEXT:       X86 WinAlloca Expander
+; CHECK-NEXT:       Eliminate PHI nodes for register allocation
+; CHECK-NEXT:       Two-Address instruction pass
+; CHECK-NEXT:       Fast Register Allocator
+; CHECK-NEXT:       Bundle Machine CFG Edges
+; CHECK-NEXT:       X86 FP Stackifier
+; CHECK-NEXT:       Prologue/Epilogue Insertion & Frame Finalization
+; CHECK-NEXT:       Post-RA pseudo instruction expansion pass
+; CHECK-NEXT:       X86 pseudo instruction expansion pass
+; CHECK-NEXT:       Analyze Machine Code For Garbage Collection
+; CHECK-NEXT:       X86 vzeroupper inserter
+; CHECK-NEXT:       Contiguously Lay Out Funclets
+; CHECK-NEXT:       StackMap Liveness Analysis
+; CHECK-NEXT:       Live DEBUG_VALUE analysis
+; CHECK-NEXT:       Insert fentry calls
+; CHECK-NEXT:       MachineDominator Tree Construction
+; CHECK-NEXT:       Machine Natural Loop Construction
+; CHECK-NEXT:       Insert XRay ops
+; CHECK-NEXT:       Implement the 'patchable-function' attribute
+; CHECK-NEXT:       Lazy Machine Block Frequency Analysis
+; CHECK-NEXT:       Machine Optimization Remark Emitter
+; CHECK-NEXT:       MachineDominator Tree Construction
+; CHECK-NEXT:       Machine Natural Loop Construction
+; CHECK-NEXT:       X86 Assembly Printer
+; CHECK-NEXT:       Free MachineFunction
+
+define void @f() {
+  ret void
+}