[llvm] [SPIR-V] Move ASSIGN_TYPE generation to PostLegalizer (PR #169696)

[Steven Perron via llvm-commits]

s-perron wrote:

FYI: @farzonl Once this PR is merged, I'll be able to add more legalization rules to handle the element-wise matrix operations.

https://github.com/llvm/llvm-project/pull/169696