Add early-stage optimization crate (#556)

* Add simple constant folding and folding tests

cranelift/filetests/simple_preopt/simplify.clif

@@ -0,0 +1,80 @@
+test simple_preopt
+target i686
+
+function %iadd_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = iadd v0, v1
+    return v2
+}
+; sameln: function %iadd_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, 2
+; nextln:     return v2
+; nextln: }
+
+function %isub_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = isub v0, v1
+    return v2
+}
+; sameln: function %isub_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, -2
+; nextln:     return v2
+; nextln: }
+
+function %icmp_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = icmp slt v0, v1
+    v3 = bint.i32 v2
+    return v3
+}
+; sameln: function %icmp_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = icmp_imm slt v0, 2
+; nextln:     v3 = bint.i32 v2
+; nextln:     return v3
+; nextln: }
+
+function %brz_bint(i32) {
+ebb0(v0: i32):
+    v3 = icmp_imm slt v0, 0
+    v1 = bint.i32 v3
+    v2 = select v1, v1, v1
+    trapz v1, user0
+    brz v1, ebb1
+    jump ebb2
+
+ebb1:
+    return
+
+ebb2:
+    return
+}
+; sameln: function %brz_bint
+; nextln: (v0: i32):
+; nextln:    v3 = icmp_imm slt v0, 0
+; nextln:    v1 = bint.i32 v3
+; nextln:    v2 = select v3, v1, v1
+; nextln:    trapz v3, user0
+; nextln:    brz v3, ebb1
+; nextln:    jump ebb2
+
+function %irsub_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = isub v1, v0
+    return v2
+}
+; sameln: function %irsub_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = irsub_imm v0, 2
+; nextln:     return v2
+; nextln: }