[test] [llvm] Add assert to builder

[hughperkins]

Issue: #

Brief Summary

Add assert to IR builder. Use like:

builder.create_assert(bool_true, "assertion failed");

Example IR:

               kernel {
                 <u1> $0 = const true
                 1 : assert $0, "assertion failed"
               }

Will facilitate creating unit test for CFG issue.

Note: this depends on the PR that adds a boolean constant to ir builder.

copilot:summary

Walkthrough

copilot:walkthrough

[feisuzhu]

/rebase

[hughperkins]

Empirically, looks like I've stopped using the Builder, e.g. https://github.com/hughperkins/taichi/blob/45cbcd5ec8601c65138c900ed271b24ac1843a19/tests/cpp/ir/control_flow_graph_test.cpp#L72-L109 I'll close this PR (If I need the Builder assert (or if someone else does) I'll re-open it).