Also, say "guard-offset pages" rather than just "guard pages" to describe the
region of a heap which is never accessible and which exists to support
optimizations for heap accesses with offsets.
And, introduce a `Uimm64` immediate type, and make all heap fields use
`Uimm64` instead of `Imm64` since they really are unsigned.
* Reorganize the global value kinds.
This:
- renames "deref" global values to "load" and gives it a offset that works
like the "load" instructions' does
- adds an explicit "iadd_imm" global value kind, which replaces the
builtin iadd in "vmctx" and "deref" global values.
- also renames "globalsym" to "symbol"
This makes several changes:
- It adds an index_type to heap declarations, allowing heaps to specify the
type for indexing. This also anticipates 64-bit heap support.
- It adds a memory_type to deref global values, allowing deref globals to
have types other than pointers. This is used to allow the bound variable
in dynamic heaps to have type i32, to match the index type in heaps
with i32 index type.
- And, it fixes heap legalization to do the bounds check in the heap's
index type.
* Remove reserved_reg functionality.
This wasn't implemented, and if we need it in the future, it seems like
it would be better to extend the concept of global values to cover this.
* Use GlobalValue::reserved_value() for sentinal values.
