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

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.

Differential Revision: https://reviews.llvm.org/D90289

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

@@ -53,7 +53,7 @@ static cl::opt<unsigned> LbrSamplingPeriod(
 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: