(MC) <target>RegisterInfo.td: alternate register names

Folks, Please confirm or correct the following assertions:

In Target.td one of the data member fields for class Register is list AltNames.

If this is for alternate names for a given register (in Mips $28 and $gp are the same) it would be quite useful for the llvm-mc assembler which has to handle cases where there are multiple names for the same register.

A quick recursive grep shows that no target is currently using this. It is also not use when generating GenAsmMatcher.inc.

If this is the case, I will try and incorporate AltNames so the strings get correctly matched in the AsmMatcher.