It's pretty much HLSL with some minor modifications (e.g. most HLSL shaders compile as-is).

It's a meta-language that compiles down to SPIR-V (edit: or other high level shader languages), if you already have your own solution for targeting those then you can just continue as you were. You're not forced to use it.

It's not just a spir-v frontend, it also transpiles to other shader languages.