[tint] Refactor internal immediate handling

Use an enum to identify internal immediates, instead of keying the map
with the byte offset.

Introduce a HasImmediate() helper that transforms can use to check if
a particular immediate has been set up.

This means that transforms that use internal immediates do not need to
be passed the offset in order to look up the index, which simplifies
various transform config structs.

Change-Id: I02a0e49139b99e700a783e2153bbf00f6325a534
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/334718
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>