Cranelift: Remove ifcmp_sp opcode. (#4578)

This was temporarily added back in #3502 due to a need from Lucet; now
that Lucet is EOL, the opcode is no longer needed and we can remove it.

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

@@ -1,34 +0,0 @@
-test compile precise-output
-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
-}
-
-;   pushq   %rbp
-;   movq    %rsp, %rbp
-; block0:
-;   cmpq    %rsp, %rdi
-;   jnbe    label1; j label2
-; block1:
-;   xorl    %eax, %eax, %eax
-;   movq    %rbp, %rsp
-;   popq    %rbp
-;   ret
-; block2:
-;   movl    $1, %eax
-;   movq    %rbp, %rsp
-;   popq    %rbp
-;   ret
-