[codegen] Check for downcasting in bitcast instruction

Bitcasting to a smaller size is invalid.

closes #854

cranelift/filetests/filetests/verifier/bitcast.clif

@@ -0,0 +1,23 @@
+test verifier
+
+; bitcast between two types of equal size if ok
+function %valid_bitcast1(i32) -> f32 { ; Ok
+ebb0(v0: i32):
+    v1 = bitcast.f32 v0
+    return v1
+}
+
+; bitcast to a type larger than the operand is ok
+function %valid_bitcast2(i32) -> i64 { ; Ok
+ebb0(v0: i32):
+    v1 = bitcast.i64 v0
+    return v1
+}
+
+; bitcast to a smaller type is not ok
+function %bad_bitcast(i64) -> i32 {
+ebb0(v0: i64):
+    v1 = bitcast.i32 v0 ; error: The bitcast argument v0 doesn't fit in a type of 32 bits
+    return v1
+}
+