Fix Xtensa reachable assert.

The immediate given to decodeImm8_sh8Operand has been already shifted.
Checking if it is an 8bit value fails therefore.

Author: Rot127

llvm/lib/Target/Xtensa/Disassembler/XtensaDisassembler.cpp

@@ -157,8 +157,8 @@ static DecodeStatus decodeImm8Operand(MCInst &Inst, uint64_t Imm,
 static DecodeStatus decodeImm8_sh8Operand(MCInst &Inst, uint64_t Imm,
                                           int64_t Address,
                                           const void *Decoder) {
-  assert(isUInt<8>(Imm) && "Invalid immediate");
-  Inst.addOperand(MCOperand::createImm(SignExtend64<16>(Imm << 8)));
+  assert(isUInt<16>(Imm) && "Invalid immediate");
+  Inst.addOperand(MCOperand::createImm(SignExtend64<16>(Imm)));
   return MCDisassembler::Success;
 }