Previously, the ir_call functions for builtin functions were replaced
with the inline implementation immediately after being added to the
instruction list. This patch replaces that with a separate pass that
lowers them after the conversion from AST to IR is complete. This will
be useful to be able to insert some handling for the precision lowering
pass before the inlining. This needs to happen because the precision
of the operations in the inlined implementation depends on the highest
precision of all of the arguments to the call.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <!3885>