arch-x86: fix CondInst decoding for MOV to Control Registers

MOV Rd,Cd is MR encoded but the control register is operand 2
not operand 1 hence this needs to be MODRM_REG not MODRM_RM.
While MOV Cd,Rd is RM encoded registers are also swapped, so
it also needs to be MODRM_REG as well (as it already correctly is).

This fixes incorrect UD2 reportings leading to invalid traps
reported in O3 on X86 FS introduced with 4e939a7 .

Change-Id: Ib33c8ba87b00e0264d33da44fff64ed9e4d2d9d8
Reviewed-on: https://gem5-review.googlesource.com/4861
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

src/arch/x86/isa/decoder/two_byte_opcodes.isa

@@ -361,7 +361,7 @@
                 // no prefix
                 0x0: decode OPCODE_OP_BOTTOM3 {
                     0x0: CondInst::MOV(
-                        {{isValidMiscReg(MISCREG_CR(MODRM_RM))}},Rd,Cd);
+                        {{isValidMiscReg(MISCREG_CR(MODRM_REG))}},Rd,Cd);
                     0x1: MOV(Rd,Dd);
                     0x2: CondInst::MOV(
                         {{isValidMiscReg(MISCREG_CR(MODRM_REG))}},Cd,Rd);