[BPF] turn on -mattr=+alu32 for cpu version v3 and later

[yonghong-song]

Summary

-mattr=+alu32 has shown good performance vs. without this attribute.
Based on discussion at

https://lore.kernel.org/bpf/[e-mail]/T/#t

cpu version v3 should support -mattr=+alu32.
This patch enabled alu32 if cpu version is v3, either specified by user
or probed by the llvm.

llvm/lib/Target/BPF/BPFSubtarget.cpp

if (CPU == "generic" || CPU == "v1")
     return;
   if (CPU == "v2") {
     HasJmpExt = true;
     return;
   }
   if (CPU == "v3") {
     HasJmpExt = true;
     HasJmp32 = true;
+    HasAlu32 = true;
     return;
   }
 }
 
 BPFSubtarget::BPFSubtarget(const Triple &TT, const std::string &CPU,
                            const std::string &FS, const TargetMachine &TM)
     : BPFGenSubtargetInfo(TT, CPU, FS), InstrInfo(),
       FrameLowering(initializeSubtargetDependencies(CPU, FS)),
       TLInfo(TM, *this) {}

llvm/test/CodeGen/BPF/32-bit-subreg-alu.ll

 ; RUN: llc -O2 -march=bpfel -mattr=+alu32 < %s | FileCheck %s
+; RUN: llc -O2 -march=bpfel -mcpu=v3 < %s | FileCheck %s
 ;
 ; int mov(int a)
 ; {
 ;   return a;
 ; }
 ;
 ; int mov_ri(void)
 ; {