Come up with a better scheme for fixed-register usages

[cfallin]

In many cases, a regalloc2 client will want to generate instructions that refer to fixed registers for uses/defs. There are multiple kinds of these register references: (i) uses/defs of a vreg that needs to be in a certain fixed register just at one instruction (e.g. for a call argument or return value), (ii) uses/defs of a machine register that is not allocatable but is used in an operand slot that can also take allocatable registers.

The former is well-covered by OperandConstraint::FixedReg, but the latter is interesting; for now our answer is to use pinned vregs, but we should have a better way of "passing through" non-allocatable registers, and we should clearly define what the semantics are.