This Fixes the slow downs I observed in techmap.v, which we
attempted to fix via the simplify ast.h route originally. This
turned out to be rather complex though.
By moving $pmux to simplemap we can just avoid that code. My
test case now runs in 310s which is 40s faster than the baseline
change.
B:507898959
Signed-off-by: Ethan Mahintorabi <ethanmoon@google.com>