Mips: Force 64bit subtarget feature to be set for ABI options (#157446)

Prepare to use this with HwMode. This is mostly code copied from x86.

Mips has an exceptionally broken system where the target-abi option
can be used to change the pointer size. i.e., you can mix and match
32-bit base triples with an explicit request to use 32-bit or 64-bit
pointers such that you cannot rely on the triple reported pointer size.
This hack manages to only work for codegen. The MC subtarget
constructors
do not have access to the target-abi name so those will continue to not
have the appropriate feature set.

Author: arsenm

llvm/lib/Target/Mips/MipsSubtarget.cpp

@@ -245,10 +245,20 @@ CodeGenOptLevel MipsSubtarget::getOptLevelToEnablePostRAScheduler() const {
 MipsSubtarget &
 MipsSubtarget::initializeSubtargetDependencies(StringRef CPU, StringRef FS,
                                                const TargetMachine &TM) {
-  StringRef CPUName = MIPS_MC::selectMipsCPU(TM.getTargetTriple(), CPU);
+  const Triple &TT = TM.getTargetTriple();
+  StringRef CPUName = MIPS_MC::selectMipsCPU(TT, CPU);
+
+  std::string FullFS;
+  if (getABI().ArePtrs64bit()) {
+    FullFS = "+ptr64";
+    if (!FS.empty())
+      FullFS = (Twine(FullFS) + "," + FS).str();
+  } else {
+    FullFS = FS.str();
+  }
 
   // Parse features string.
-  ParseSubtargetFeatures(CPUName, /*TuneCPU*/ CPUName, FS);
+  ParseSubtargetFeatures(CPUName, /*TuneCPU=*/CPUName, FullFS);
   // Initialize scheduling itinerary for the specified CPU.
   InstrItins = getInstrItineraryForCPU(CPUName);