[X86] Define segment MI operands as regs instead of i8imm.

We've been pretending that segments are i8imm since the initial
support (r68645), predating the addition of the SEGMENT_REG class
(r81895).  That happens to works, but is wrong, and inconsistent
with how we print (e.g., X86ATTInstPrinter::printMemReference)
and parse them (e.g., X86Operand::addMemOperands).

This change shouldn't affect any tool users, but is visible to
library users or out-of-tree tablegen backends: this causes
MCOperandInfo for the segment op to have an RC instead of "unknown",
and TII::getRegClass to actually return something.  As the registers
are reserved and no vregs of the class ever created, that shouldn't
change anything.

No test change; no suspicious getRegClass() in X86 and CodeGen.

llvm-svn: 271559