[asm parser] Add support for predicating MnemonicAlias based on the assembler
variant/dialect.  Addresses a FIXME in the emitMnemonicAliases function.
Use and test case to come shortly.
rdar://13688439 and part of PR13340.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179804 91177308-0d34-0410-b5e6-96231b3b80d8