Stablize MCK_Reg ordering in AsmMatcherEmitter

clang bootstraps intermittently failed for me due a difference in
the MCK_Reg ordering in ARMGenAsmMatcher.inc.  E.g. in my latest
run the stage 1 and stage 3 versions were the same but the stage 2
one was different (though still functionally correct).  This meant
that the .o comparison failed.

MCK_Regs were assigned by iterating over a std::set< std::set<Record*> >,
and since std::set is sorted lexicographically, the order depended on the
order of the pointer values.  This patch replaces the pointer ordering
with LessRecordByID.