[X86] Remove encoding value from the X86_FEATURE and X86_FEATURE_COMPAT macro. NFCI

This was orignally done so we could separate the compatibility
values and the llvm internal only features into a separate entries
in the feature array. This was needed when we explicitly had to
convert the feature into the proper 32-bit chunk at every reference
and we didn't want things moving around.

Now everything is in an array and we have helper funtions or macros
to convert encoding to index. So we renumbering is no longer an
issue.