[GISel]: Print useful remarks when GISelAbort = 1

[aditya_nandakumar]

Summary

Currently when abort is enabled, we get a diagnostic saying "Fallback path used .... " and the program terminates. To actually figure out what the reason is, we need to run again with another verbose argument "-pass-remarks-missed=gisel".
Because we know we're going to call report_fatal_error(), this patch prints the reason for fallback and this is a lot more useful than the warning diagnostic above. Also, this won't really be a compile time issue.

[aemerson]

Seems reasonable.

[aditya_nandakumar]

Thanks. r326215

lib/CodeGen/GlobalISel/Utils.cpp

 void llvm::reportGISelFailure(MachineFunction &MF, const TargetPassConfig &TPC,
                               MachineOptimizationRemarkEmitter &MORE,
                               const char *PassName, StringRef Msg,
                               const MachineInstr &MI) {
   MachineOptimizationRemarkMissed R(PassName, "GISelFailure: ",
                                     MI.getDebugLoc(), MI.getParent());
   R << Msg;
   // Printing MI is expensive;  only do it if expensive remarks are enabled.
-  if (MORE.allowExtraAnalysis(PassName))
+  if (TPC.isGlobalISelAbortEnabled() || MORE.allowExtraAnalysis(PassName))
     R << ": " << ore::MNV("Inst", MI);
   reportGISelFailure(MF, TPC, MORE, R);
 }
 
 Optional<int64_t> llvm::getConstantVRegVal(unsigned VReg,
                                            const MachineRegisterInfo &MRI) {
   MachineInstr *MI = MRI.getVRegDef(VReg);
   if (MI->getOpcode() != TargetOpcode::G_CONSTANT)