[PPC] Add Subtarget feature slower ISEL targets. NFC

[iteratee]

Summary

Power7 and Power8 have isel instructions that are slow compared to a
predicted branch. Document this as a subtarget feature.

[hfinkel]

I think it would be best to make HasISEL a { Fast, Slow, No }-type enum. We can have separate feature strings for the llc command line, but otherwise it becomes difficult to make sure that the -misel -mno-isel options in Clang override things in the correct way. Please experiment with that -- that's the underlying design constraint.

lib/Target/PowerPC/PPC.td

90

If this line is longer that 80 characters, please unindent it as with other similar lines.

lib/Target/PowerPC/PPC.td

 def FeatureLFIWAX    : SubtargetFeature<"lfiwax","HasLFIWAX", "true",
                                         "Enable the lfiwax instruction">;
 def FeatureFPRND     : SubtargetFeature<"fprnd", "HasFPRND", "true",
                                         "Enable the fri[mnpz] instructions">;
 def FeatureFPCVT     : SubtargetFeature<"fpcvt", "HasFPCVT", "true",
   "Enable fc[ft]* (unsigned and single-precision) and lfiwzx instructions">;
 def FeatureISEL      : SubtargetFeature<"isel","HasISEL", "true",
                                         "Enable the isel instruction">;
+def FeatureSlowISEL  : SubtargetFeature<"slowisel","HasSlowISEL", "true",
+                                        "The ISEL instruction is slow compared to a predicted branch">;
 def FeaturePOPCNTD   : SubtargetFeature<"popcntd","HasPOPCNTD", "true",
                                         "Enable the popcnt[dw] instructions">;
 def FeatureBPERMD    : SubtargetFeature<"bpermd", "HasBPERMD", "true",
                                         "Enable the bpermd instruction">;
 def FeatureExtDiv    : SubtargetFeature<"extdiv", "HasExtDiv", "true",
                                         "Enable extended divide instructions">;
 def FeatureLDBRX     : SubtargetFeature<"ldbrx","HasLDBRX", "true",
                                         "Enable the ldbrx instruction">;
/*  Since new processors generally contain a superset of features of those that
     used elsewhere.
 */
 def ProcessorFeatures {
   list<SubtargetFeature> Power7FeatureList =
       [DirectivePwr7, FeatureAltivec, FeatureVSX,
        FeatureMFOCRF, FeatureFCPSGN, FeatureFSqrt, FeatureFRE,
        FeatureFRES, FeatureFRSQRTE, FeatureFRSQRTES,
        FeatureRecipPrec, FeatureSTFIWX, FeatureLFIWAX,
-       FeatureFPRND, FeatureFPCVT, FeatureISEL,
+       FeatureFPRND, FeatureFPCVT, FeatureISEL, FeatureSlowISEL,
        FeaturePOPCNTD, FeatureCMPB, FeatureLDBRX,
        Feature64Bit /*, Feature64BitRegs */,
        FeatureBPERMD, FeatureExtDiv,
        FeatureMFTB, DeprecatedDST];
   list<SubtargetFeature> Power8SpecificFeatures =
       [DirectivePwr8, FeatureP8Altivec, FeatureP8Vector, FeatureP8Crypto,
        FeatureHTM, FeatureDirectMove, FeatureICBT, FeaturePartwordAtomic,
        FeatureFusion];

lib/Target/PowerPC/PPCSubtarget.h

protected:
   bool HasFSQRT;
   bool HasFRE, HasFRES, HasFRSQRTE, HasFRSQRTES;
   bool HasRecipPrec;
   bool HasSTFIWX;
   bool HasLFIWAX;
   bool HasFPRND;
   bool HasFPCVT;
   bool HasISEL;
+  bool HasSlowISEL;
   bool HasPOPCNTD;
   bool HasBPERMD;
   bool HasExtDiv;
   bool HasCMPB;
   bool HasLDBRX;
   bool IsBookE;
   bool HasOnlyMSYNC;
   bool IsE500;
public:
   bool hasSPE() const { return HasSPE; }
   bool hasQPX() const { return HasQPX; }
   bool hasVSX() const { return HasVSX; }
   bool hasP8Vector() const { return HasP8Vector; }
   bool hasP8Altivec() const { return HasP8Altivec; }
   bool hasP8Crypto() const { return HasP8Crypto; }
   bool hasMFOCRF() const { return HasMFOCRF; }
   bool hasISEL() const { return HasISEL; }
+  bool hasSlowISEL() const { return HasSlowISEL; }
   bool hasPOPCNTD() const { return HasPOPCNTD; }
   bool hasBPERMD() const { return HasBPERMD; }
   bool hasExtDiv() const { return HasExtDiv; }
   bool hasCMPB() const { return HasCMPB; }
   bool hasLDBRX() const { return HasLDBRX; }
   bool isBookE() const { return IsBookE; }
   bool hasOnlyMSYNC() const { return HasOnlyMSYNC; }
   bool isPPC4xx() const { return IsPPC4xx; }

lib/Target/PowerPC/PPCSubtarget.cpp

void PPCSubtarget::initializeEnvironment() {
   HasFRSQRTE = false;
   HasFRSQRTES = false;
   HasRecipPrec = false;
   HasSTFIWX = false;
   HasLFIWAX = false;
   HasFPRND = false;
   HasFPCVT = false;
   HasISEL = false;
+  HasSlowISEL = false;
   HasPOPCNTD = false;
   HasBPERMD = false;
   HasExtDiv = false;
   HasCMPB = false;
   HasLDBRX = false;
   IsBookE = false;
   HasOnlyMSYNC = false;
   IsPPC4xx = false;