Fold 'ifcmp_imm' + 'brif' where imm is zero and cond is 'eq' or 'ne', into 'brz' or 'brnz'.

cranelift/filetests/filetests/simple_preopt/branch.clif

@@ -0,0 +1,29 @@
+test simple_preopt
+target x86_64
+
+function %brif_to_brz_fold(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = ifcmp_imm v0, 0
+    brif eq v1, ebb1
+    jump ebb2
+ebb1:
+    v2 = iconst.i32 1
+    return v2
+ebb2:
+    v3 = iconst.i32 2
+    return v3
+}
+; sameln: function %brif_to_brz_fold
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = ifcmp_imm v0, 0
+; nextln:     brz v0, ebb1
+; nextln:     jump ebb2
+; nextln: 
+; nextln: ebb1:
+; nextln:     v2 = iconst.i32 1
+; nextln:     return v2
+; nextln: 
+; nextln: ebb2:
+; nextln:     v3 = iconst.i32 2
+; nextln:     return v3
+; nextln: }