cmd/compile: remove Type haspointers caches

cmd/compile: remove Type haspointers caches

Even very large Types are not very big.
The haspointer cache looks like premature optimization.
Removing them has no detectable compiler performance impact,
and it removes mutable shared state used by the backend.

Updates #15756

Change-Id: I2d2cf03f470f5eef5bcd50ff693ef6a01d481700
Reviewed-on: https://go-review.googlesource.com/38912
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>