Function generate_ir 

pub fn generate_ir(
    inputs: &CodegenInputs<'_>,
    opt_level: OptLevel,
) -> CompileResult<String>

Generate LLVM textual IR from a collection of function CFGs.

Returns the LLVM IR in human-readable .ll format. Used by --emit asm to produce inspectable IR in place of native assembly. At -O1+ the returned IR is the post-optimization form.