Add back the ifcmp_sp CLIF opcode.

This opcode was removed as part of the old-backend cleanup in #3446.
While this opcode will definitely go away eventually, it is
unfortunately still used today in Lucet (as we just discovered while
working to upgrade Lucet's pinned Cranelift version). Lucet is
deprecated and slated to eventually be completely sunset in favor of
Wasmtime; but until that happens, we need to keep this opcode.

cranelift/filetests/filetests/isa/x64/ifcmp-sp.clif

@@ -0,0 +1,31 @@
+test compile
+target x86_64
+
+function %f(i64) -> i32 {
+block0(v0: i64):
+    v1 = ifcmp_sp v0
+    brif ugt v1, block1
+    jump block2
+
+block1:
+    v2 = iconst.i32 0
+    return v2
+
+block2:
+    v3 = iconst.i32 1
+    return v3
+}
+
+; check:  pushq   %rbp
+; nextln: movq    %rsp, %rbp
+; nextln: cmpq    %rsp, %rdi
+; nextln: jnbe    label1; j label2
+; check:  xorl    %eax, %eax
+; nextln: movq    %rbp, %rsp
+; nextln: popq    %rbp
+; nextln: ret
+; check:  movl    $$1, %eax
+; nextln: movq    %rbp, %rsp
+; nextln: popq    %rbp
+; nextln: ret
+