Removing "enum" from "enum machine_mode"

[Richard Sandiford <richard dot sandiford at arm dot com>]

I have some plans to "clean up" the machine_mode handling and perhaps
make it hierarchical, so that functions that can only handle scalar
integer modes (say) will be able to take a scalar_int_mode rather than
a machine_mode as argument.  The first step would be to do a blanket
removal of the (in C++) redundant "enum" from all those "enum machine_mode"
variables, parameters and fields.

Regardless of whether the hierarchy sounds like a good idea, would removing
the "enum" be OK?  There's never a good time for that much churn, but since
the release branches are fairly mature and since we already have quite a
bit of churn in rtl land between 4.9 and 5, now seemed like as good a
time as any.

Thanks,
Richard