Reduction pass idea: replace expression with sub-expression

Suggested by @alan-baker in https://github.com/KhronosGroup/SPIRV-Tools/pull/2099:

"For each result id, iterate through its users. If the user's type matches the candidate type and the user is not an OpPhi then replace that whole instruction with the candidate (either replacing all uses carefully or by inserting an OpCopyObject). This would effectively replace an expression with a subexpression."