Verify that the instruction encoding matches what the ISA would encode.

Fixes #69

cranelift/filetests/isa/riscv/verify-encoding.cton

@@ -0,0 +1,21 @@
+test verifier
+isa riscv
+
+function RV32I(i32 link [%x1]) -> i32 link [%x1] {
+    fn0 = function foo()
+
+ebb0(v9999: i32):
+    ; iconst.i32 needs legalizing, so it should throw a
+    [R#0,-]         v1 = iconst.i32 1 ; error: Instruction failed to re-encode
+    return v9999
+}
+
+function RV32I(i32 link [%x1]) -> i32 link [%x1] {
+    fn0 = function foo()
+
+ebb0(v9999: i32):
+    v1 = iconst.i32 1
+    v2 = iconst.i32 2
+    [R#0,-]         v3 = iadd v1, v2 ; error: Instruction re-encoding
+    return v9999
+}