ISAC: Minor changes to VHDL-93 IRs

I have made very minor changes to about 12 IRs in the
vhdl 93 directory on eda.org: /pub/isac/IRs-VHDL-93

The changes are to IRs which supersede VHDL-87 IRs,
and make sure that the superseded IR is referenced in the
new IR.

I've done this because, in the not too distant future,
we'll need to start looking at VHDL-93 IRs and determine
which ones are still open.

Chuck Swart