[TableGen] Fix sort order of asm operand classes

This is a fix for https://llvm.org/bugs/show_bug.cgi?id=22796.

The previous implementation of ClassInfo::operator< allowed cycles of classes
such that x < y < z < x, meaning that a list of them cannot be correctly
sorted, and the sort order could differ with different standard libraries.

The original implementation sorted classes by ValueName if they were otherwise
equal. This isn't strictly necessary, but some backends seem to accidentally
rely on it. If I reverse this comparison I get 8 test failures spread across
the AArch64, Mips and X86 backends, so I have left it in until those backends
can be fixed.

There was one case in the X86 backend where the observable behaviour of the
assembler is changed by this patch. This was because some of the memory asm
operands were not marked as children of X86MemAsmOperand.

Differential Revision: http://reviews.llvm.org/D16141

llvm-svn: 258677