[X86] Specify all TSFlags bit-offsets symbolically

No functional change.

The offsets for the other bitfields are specified symbolically.  I need to
increase the size for one of the earlier fields which is easier after this
cleanup.

Why these bits are relative to VEXShift is a bit strange but that is for
another cleanup.

I made sure that the values for the enums are unchanged after this change.

llvm-svn: 213011