[BPF] Refactor BPFSubtarget::initSubtargetFeatures() (NFC)

Refactor it to make it match the style of
BPFTargetInfo::getTargetDefines(), so that when we add -mcpu=v5 in the
future, we can simply append e.g.:

  if (CpuVerNum >= 5)
    HasFoo = true;

instead of saying:

  if (CPU == "v5") {
    HasJmpExt = true;
    HasJmp32 = true;
    HasAlu32 = true;
    HasLdsx = !Disable_ldsx;
    HasMovsx = !Disable_movsx;
    HasBswap = !Disable_bswap;
    HasSdivSmod = !Disable_sdiv_smod;
    HasGotol = !Disable_gotol;
    HasStoreImm = !Disable_StoreImm;
    HasFoo = true;
  }

This also makes it clearer that newer "CPU"s always support older
features.  No functional changes intended.

Author: peilin-ye

llvm/lib/Target/BPF/BPFSubtarget.cpp

@@ -69,29 +69,25 @@ void BPFSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
     CPU = "v3";
   if (CPU == "probe")
     CPU = sys::detail::getHostCPUNameForBPF();
-  if (CPU == "generic" || CPU == "v1")
+  if (CPU.empty() || CPU == "generic" || CPU == "v1")
     return;
-  if (CPU == "v2") {
-    HasJmpExt = true;
-    return;
-  }
-  if (CPU == "v3") {
+
+  int CpuVerNum = CPU.back() - '0';
+  if (CpuVerNum >= 2)
     HasJmpExt = true;
+
+  if (CpuVerNum >= 3) {
     HasJmp32 = true;
     HasAlu32 = true;
-    return;
   }
-  if (CPU == "v4") {
-    HasJmpExt = true;
-    HasJmp32 = true;
-    HasAlu32 = true;
+
+  if (CpuVerNum >= 4) {
     HasLdsx = !Disable_ldsx;
     HasMovsx = !Disable_movsx;
     HasBswap = !Disable_bswap;
     HasSdivSmod = !Disable_sdiv_smod;
     HasGotol = !Disable_gotol;
     HasStoreImm = !Disable_StoreImm;
-    return;
   }
 }