Remove the HeapLoad/HeapStore instruction formats.

These formats are not used any longer after the heap_load and heap_store
instructions were replaced by heap_addr.

Also drop the Uoffset32 immediate operand type which isn't used either.

lib/cretonne/src/verifier/mod.rs

@@ -335,8 +335,6 @@ impl<'a> Verifier<'a> {
             IntCompare { .. } |
             IntCompareImm { .. } |
             FloatCompare { .. } |
-            HeapLoad { .. } |
-            HeapStore { .. } |
             Load { .. } |
             Store { .. } |
             RegMove { .. } => {}