Add an ifcmp_sp instruction.

This will be used to implement the stack_check macro.

cranelift/docs/langref.rst

@@ -947,6 +947,7 @@ The prologue and epilogue of a function needs to manipulate special registers li
 pointer and the frame pointer. These instructions should not be used in regular code.
 
 .. autoinst:: adjust_sp_imm
+.. autoinst:: ifcmp_sp
 .. autoinst:: copy_special
 
 .. _extload-truncstore:

lib/cretonne/meta/base/instructions.py

@@ -579,6 +579,17 @@ adjust_sp_imm = Instruction(
     ins=(StackOffset,),
     other_side_effects=True)
 
+f = Operand('f', iflags)
+
+ifcmp_sp = Instruction(
+    'ifcmp_sp', r"""
+    Compare ``addr`` with the stack pointer and set the CPU flags.
+
+    This is like :inst:`ifcmp` where ``addr`` is the LHS operand and the stack
+    pointer is the RHS.
+    """,
+    ins=addr, outs=f)
+
 regspill = Instruction(
         'regspill', r"""
         Temporarily divert ``x`` from ``src`` to ``SS``.