TableGen: convert InstAlias's Emit bit to an int.

When multiple aliases overlap, the correct string to print can often be
determined purely by considering the InstAlias declarations in some particular
order. This allows the user to specify that order manually when desired,
without resorting to hacking around with the default lexicographical order on
Record instantiation, which is error-prone and ugly.

I was also mistaken about "add w2, w3, w4" being the same as "add w2, w3, w4,
uxtw". That's only true if Rn is the stack pointer.