Add an isa::StackRef type.

This contains encoding details for a stack reference: The base register
and offset to use in the specific instruction encoding.

Generate StackRef objects called in_stk0 etc for the binemit recipe
code. All binemit recipes need to compute base pointer offsets for stack
references, so have the automatically generated code do it.

lib/cretonne/src/isa/riscv/binemit.rs

@@ -2,7 +2,7 @@
 
 use binemit::{CodeSink, Reloc, bad_encoding};
 use ir::{Function, Inst, InstructionData};
-use isa::RegUnit;
+use isa::{RegUnit, StackRef, StackBaseMask};
 use predicates::is_signed_int;
 use regalloc::RegDiversions;