+/// GetRegistersForValue - Assign registers (virtual or physical) for the
+/// specified operand. We prefer to assign virtual registers, to allow the
+/// register allocator handle the assignment process. However, if the asm uses
+/// features that we can't model on machineinstrs, we have SDISel do the
+/// allocation. This produces generally horrible, but correct, code.
+///
+/// OpInfo describes the operand.
+/// HasEarlyClobber is true if there are any early clobber constraints (=&r)
+/// or any explicitly clobbered registers.
+/// Input and OutputRegs are the set of already allocated physical registers.
+///