[llvm] [SPIRV] Fix SPV_KHR_expect_assume support (PR #67793)

[Paulo Matos via llvm-commits]

pmatos wrote:

This is a draft because I noticed that if we do not enable the use of the extension, extension and capability are not declared but instructions are still generated. What's the correct behaviour for this? I would think that since assume and expect do not alter the semantics we can just not issue them. The alternative is to generate an error.

What do you think? Shall we just ignore the instruction if we cannot use the extension?

https://github.com/llvm/llvm-project/pull/67793