[llvm] [SPIR-V] Replace assert with report_fatal (PR #118617)

[Nathan Gauër via llvm-commits]

Keenuts wrote:


> I think the correct way to deal with any case where this method doesn't work is to return without changing anything and pass info about the fail of sortBlocks()

Note that since https://github.com/llvm/llvm-project/pull/116996 the visitor is not used for block sorting anymore. It's use-case is only in the structurizer which has tighter requirements than `sortBlocks`

https://github.com/llvm/llvm-project/pull/118617