peepmatic: Make peepmatic optional to enable

Rather than outright replacing parts of our existing peephole optimizations
passes, this makes peepmatic an optional cargo feature that can be enabled. This
allows us to take a conservative approach with enabling peepmatic everywhere,
while also allowing us to get it in-tree and make it easier to collaborate on
improving it quickly.

cranelift/filetests/filetests/peepmatic/simplify32.clif

@@ -0,0 +1,60 @@
+test peepmatic
+target i686
+
+;; 32-bits platforms.
+
+function %iadd_imm(i32) -> i32 {
+block0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = iadd v0, v1
+    return v2
+}
+; sameln: function %iadd_imm
+; nextln: block0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, 2
+; nextln:     return v2
+; nextln: }
+
+function %isub_imm(i32) -> i32 {
+block0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = isub v0, v1
+    return v2
+}
+; sameln: function %isub_imm
+; nextln: block0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, -2
+; nextln:     return v2
+; nextln: }
+
+function %icmp_imm(i32) -> i32 {
+block0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = icmp slt v0, v1
+    v3 = bint.i32 v2
+    return v3
+}
+; sameln: function %icmp_imm
+; nextln: block0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = icmp_imm slt v0, 2
+; nextln:     v3 = bint.i32 v2
+; nextln:     return v3
+; nextln: }
+
+;; Don't simplify operations that would get illegal because of lack of native
+;; support.
+function %iadd_imm(i64) -> i64 {
+block0(v0: i64):
+    v1 = iconst.i64 2
+    v2 = iadd v0, v1
+    return v2
+}
+; sameln: function %iadd_imm
+; nextln: block0(v0: i64):
+; nextln:     v1 = iconst.i64 2
+; nextln:     v2 = iadd v0, v1
+; nextln:     return v2
+; nextln: }