MOV instructions 8C and 8E can be prefixed with a REX prefix to extend
the source/destination register.
However, the R bit in REX will be applied to the segment register.
The decoder file checks for valid segment registers, checking the
MODRM_REG only, however, later this will be extended with the REX_R when
adding the register to the sources/destinations of the instruction.
This will trigger an assert.
Additionally, MOV instructions of various miscelaneous registers are
also not check for being valid when taking into account the REX_R bit.
This patch checks that the REX_R is not set, otherwise, UD2 will be
generated.
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