This will eventually replace Shader and related subclasses. The
interesting bit here is the instruction iterator, which allows fairly
safe access to the instruction stream without needing the rest of the
code to care too much about the physical layout.

Bug: b/120799499

Change-Id: Id0d94c4b807ddb1e4325de147ca1f651171779b7
Reviewed-on: https://swiftshader-review.googlesource.com/c/23049Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>