[llvm] [TableGen] Use buildConstant to emit apply pattern immediates (PR #66077)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> > This adds flexibility, e.g. it allows us to seamlessly handle vector cases, where a G_BUILD_VECTOR is needed to create a splat.
> 
> Do you have any follow ups that would take advantage of that?

I got a couple more patches in this stack, there is an example in #66079 which isn't possible without this 

https://github.com/llvm/llvm-project/pull/66077