[llvm-exegesis] Do not silently fail on unknown instruction encoding formats.

[courbet]

Summary

The addition of TILELOADD instructions with a new encoding format
triggered a hard abort instead of proper error reporting due to the use
of llvm_unreachable for actually reachable code.
Properly report an error when the encoding format is unknown.

llvm/tools/llvm-exegesis/lib/X86/Target.cpp

 
 // FIXME: Validates that repetition-mode is loop if LBR is requested.
 
 // Returns a non-null reason if we cannot handle the memory references in this
 // instruction.
 static const char *isInvalidMemoryInstr(const Instruction &Instr) {
   switch (Instr.Description.TSFlags & X86II::FormMask) {
   default:
-    llvm_unreachable("Unknown FormMask value");
+    return "Unknown FormMask value";
   // These have no memory access.
   case X86II::Pseudo:
   case X86II::RawFrm:
   case X86II::AddCCFrm:
   case X86II::PrefixByte:
   case X86II::MRMDestReg:
   case X86II::MRMSrcReg:
   case X86II::MRMSrcReg4VOp3: