[RFC][Patch 1/3] Add a new class of predicates for variant scheduling classes.

This patch is the first of a sequence of three patches described by the LLVM-dev
RFC "MC support for variant scheduling classes".
http://lists.llvm.org/pipermail/llvm-dev/2018-May/123181.html

The goal of this patch is to introduce a new class of scheduling predicates for
SchedReadVariant and SchedWriteVariant.

An MCSchedPredicate can be used instead of a normal SchedPredicate to model
checks on the instruction (either a MachineInstr or a MCInst).
Internally, an MCSchedPredicate encapsulates an MCInstPredicate definition.
MCInstPredicate allows the definition of expressions with a well-known semantic,
that can be used to generate code for both MachineInstr and MCInst.

This is the first step toward teaching to tools like lllvm-mca how to resolve
variant scheduling classes.

Differential Revision: https://reviews.llvm.org/D46695

llvm-svn: 333282