Many other pieces will need an index of spirv-id to defining
instruction. Start building that up.

Bug: b/120799499

Change-Id: Ief6583f45c49fce02cb5dc40d12c6ba928e48fd7
Reviewed-on: https://swiftshader-review.googlesource.com/c/23428Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>