spirv: wrap push ssa/pointer values

This refactor allows for common code to apply decoration on all
ssa/pointer values. In particular this will allow to propagage access
qualifiers.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Suggested-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 86b53770)
[Lionel Landwerlin: patch adapted for 19.1 branch]