[llvm] [TableGen][GlobalISel] Add specialized opcodes (PR #74823)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> What are the code size gain here? What is the compile time impact? (I expect that reading ULEB is not going to be free.)

This doesn't add ULEB reading to the table, that's in https://github.com/llvm/llvm-project/pull/74429 
There are some numbers in the RFC. So far no noticeable impact to performance, it might even be a bit faster due to the massive reduction in table size (about 8Mb less in LLC's binary size)

https://github.com/llvm/llvm-project/pull/74823