Remove unusable variants from Align enum

I discovered today that Wasmer throws an error if you use
"higher than natural" alignment in a store instruction.
So let's take them out of the enum.

compiler/gen_wasm/src/wasm_module/code_builder.rs

@@ -84,7 +84,9 @@ impl std::fmt::Debug for VmBlock<'_> {
     }
 }
 
-/// Wasm memory alignment. (Rust representation matches Wasm encoding)
+/// Wasm memory alignment for load/store instructions.
+/// Rust representation matches Wasm encoding.
+/// It's an error to specify alignment higher than the "natural" alignment of the instruction
 #[repr(u8)]
 #[derive(Clone, Copy, Debug)]
 pub enum Align {
@@ -92,10 +94,6 @@ pub enum Align {
     Bytes2 = 1,
     Bytes4 = 2,
     Bytes8 = 3,
-    Bytes16 = 4,
-    Bytes32 = 5,
-    Bytes64 = 6,
-    // ... we can add more if we need them ...
 }
 
 impl From<u32> for Align {
@@ -105,9 +103,6 @@ impl From<u32> for Align {
             2 => Align::Bytes2,
             4 => Align::Bytes4,
             8 => Align::Bytes8,
-            16 => Align::Bytes16,
-            32 => Align::Bytes32,
-            64 => Align::Bytes64,
             _ => panic!("{:?}-byte alignment not supported", x),
         }
     }