[TableGen] Prevent double flattening of InstAlias asm strings in the asm matcher emitter.

Unlike CodeGenInstruction, CodeGenInstAlias was flatting asm strings in its constructor. For instructions it was the users responsibility to flatten the string.

AsmMatcherEmitter didn't know this and treated them the same. This caused double flattening of InstAliases. This is mostly harmless unless the desired assembly string contains curly braces. The second flattening wouldn't know to ignore these and would remove the curly braces. And for variant 1 it would remove the contents of them as well.

To mitigate this, this patch makes removes the flattening from the CodeGenIntAlias constructor and modifies AsmWriterEmitter to account for the flattening not having been done.

llvm-svn: 334919