[TableGen][AsmWriterEmitter] Use a deterministic order to sort InstrAliases

Inside an alias group, when ordering instruction aliases, we rely
on the priority field to sort them.
When the priority is not set or more generally when there is a tie between
two aliases, we used to rely on the lexicographic order. However, this
order can change for the anonymous records when more instruction, intrinsic,
etc. are inserted.

For instance, given two anonymous records r1 and r2 with respective name
A_999 and A_1000, their lexicography order will be r2 then r1. Now, if
an instruction is added before them, their name will become respectively
A_1000 and A_1001, thus the lexicography order will be r1 then r2, i.e.,
it changed.

If that happens in an alias group, the assembly output would prefer a
different alias for no apparent good reasons.

A way to fix that is to use proper priority for all aliases, but we
can also make the tie breaker comparison smarter and use a deterministic
ordering. This is what this patch does.

llvm-svn: 294695