Asserts are used in place of DXASSERT in shader debug passes

[jeffnn]

Description See bob80905's comments on PR https://github.com/microsoft/DirectXShaderCompiler/pull/6300 In summary, "assert" was used when "DXASSERT" is preferred. Plus, some of the asserts are just plain "assert(false)" and could be refactored into something more meaningful.

Steps to Reproduce (Code issue. No repro exists.)

Actual Behavior "assert" is used!

Environment None. Code issue.

[llvm-beanz]

I think it would be better to use the c-standard assert over DXASSERT for portability and understandability of the code. Cases that are assert(false) should probably be llvm_unreachable(<msg>) instead.