Allow register classes to match a containing class in InstAliases.

Allow register classes to match a containing class in InstAliases.

If the register class in the source alias is a subclass of the register class
of the actual instruction, the alias can still match OK since the constraints
are strictly a subset of what the instruction can actually handle.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143200 91177308-0d34-0410-b5e6-96231b3b80d8