[X86] Move even more of our CPU to feature mapping switch to use fallthroughs

[craig.topper]

Summary

This arranges more of the Intel and AMD CPUs into fallthrough positions based on their features. We may be able to merge this new AMD set with the BTVER or BDVER sets but I didn't look that closely.

[RKSimon]

LGTM, there are a couple of other fall-throughs that could still be done.

lib/Basic/Targets/X86.cpp

141

CK_Pentium3/CK_C3_2/CK_PentiumM/CK_Pentium4/CK_x86_64 can be put below Yonah et al. (unless you're worried about including non-Intel enums)?

lib/Basic/Targets/X86.cpp

bool X86TargetInfo::initFeatureMap(
   case CK_C3_2:
     setFeatureEnabledImpl(Features, "sse", true);
     setFeatureEnabledImpl(Features, "fxsr", true);
     break;
   case CK_PentiumM:
   case CK_Pentium4:
   case CK_x86_64:
     setFeatureEnabledImpl(Features, "sse2", true);
     setFeatureEnabledImpl(Features, "fxsr", true);
     break;
-  case CK_Yonah:
-  case CK_Prescott:
-  case CK_Nocona:
-    setFeatureEnabledImpl(Features, "sse3", true);
-    setFeatureEnabledImpl(Features, "fxsr", true);
-    setFeatureEnabledImpl(Features, "cx16", true);
-    break;
-  case CK_Core2:
-    setFeatureEnabledImpl(Features, "ssse3", true);
-    setFeatureEnabledImpl(Features, "fxsr", true);
-    setFeatureEnabledImpl(Features, "cx16", true);
-    break;
-  case CK_Penryn:
-    setFeatureEnabledImpl(Features, "sse4.1", true);
-    setFeatureEnabledImpl(Features, "fxsr", true);
-    setFeatureEnabledImpl(Features, "cx16", true);
-    break;
   case CK_Cannonlake:
     setFeatureEnabledImpl(Features, "avx512ifma", true);
     setFeatureEnabledImpl(Features, "avx512vbmi", true);
     setFeatureEnabledImpl(Features, "sha", true);
     LLVM_FALLTHROUGH;
   case CK_SkylakeServer:
     setFeatureEnabledImpl(Features, "avx512f", true);
     setFeatureEnabledImpl(Features, "avx512cd", true);
case CK_SandyBridge:
     setFeatureEnabledImpl(Features, "xsaveopt", true);
     LLVM_FALLTHROUGH;
   case CK_Westmere:
     setFeatureEnabledImpl(Features, "aes", true);
     setFeatureEnabledImpl(Features, "pclmul", true);
     LLVM_FALLTHROUGH;
   case CK_Nehalem:
     setFeatureEnabledImpl(Features, "sse4.2", true);
+    LLVM_FALLTHROUGH;
+  case CK_Penryn:
+    setFeatureEnabledImpl(Features, "sse4.1", true);
+    LLVM_FALLTHROUGH;
+  case CK_Core2:
+    setFeatureEnabledImpl(Features, "ssse3", true);
+    LLVM_FALLTHROUGH;
+  case CK_Yonah:
+  case CK_Prescott:
+  case CK_Nocona:
+    setFeatureEnabledImpl(Features, "sse3", true);
     setFeatureEnabledImpl(Features, "fxsr", true);
     setFeatureEnabledImpl(Features, "cx16", true);
     break;
   case CK_Goldmont:
     setFeatureEnabledImpl(Features, "sha", true);
     setFeatureEnabledImpl(Features, "rdrnd", true);
     setFeatureEnabledImpl(Features, "rdseed", true);
     setFeatureEnabledImpl(Features, "xsave", true);
case CK_KNL:
     setFeatureEnabledImpl(Features, "movbe", true);
     break;
   case CK_K6_2:
   case CK_K6_3:
   case CK_WinChip2:
   case CK_C3:
     setFeatureEnabledImpl(Features, "3dnow", true);
     break;
-  case CK_Athlon:
-  case CK_Geode:
-    setFeatureEnabledImpl(Features, "3dnowa", true);
-    break;
-  case CK_AthlonXP:
-    setFeatureEnabledImpl(Features, "sse", true);
-    setFeatureEnabledImpl(Features, "3dnowa", true);
-    setFeatureEnabledImpl(Features, "fxsr", true);
-    break;
-  case CK_K8:
-    setFeatureEnabledImpl(Features, "sse2", true);
-    setFeatureEnabledImpl(Features, "3dnowa", true);
-    setFeatureEnabledImpl(Features, "fxsr", true);
-    break;
   case CK_AMDFAM10:
     setFeatureEnabledImpl(Features, "sse4a", true);
     setFeatureEnabledImpl(Features, "lzcnt", true);
     setFeatureEnabledImpl(Features, "popcnt", true);
     LLVM_FALLTHROUGH;
   case CK_K8SSE3:
     setFeatureEnabledImpl(Features, "sse3", true);
-    setFeatureEnabledImpl(Features, "3dnowa", true);
+    LLVM_FALLTHROUGH;
+  case CK_K8:
+    setFeatureEnabledImpl(Features, "sse2", true);
+    LLVM_FALLTHROUGH;
+  case CK_AthlonXP:
+    setFeatureEnabledImpl(Features, "sse", true);
     setFeatureEnabledImpl(Features, "fxsr", true);
+    LLVM_FALLTHROUGH;
+  case CK_Athlon:
+  case CK_Geode:
+    setFeatureEnabledImpl(Features, "3dnowa", true);
     break;
   case CK_BTVER2:
     setFeatureEnabledImpl(Features, "avx", true);
     setFeatureEnabledImpl(Features, "aes", true);
     setFeatureEnabledImpl(Features, "pclmul", true);
     setFeatureEnabledImpl(Features, "bmi", true);
     setFeatureEnabledImpl(Features, "f16c", true);
     setFeatureEnabledImpl(Features, "xsaveopt", true);