cmd/compile: reserve less memory for bitvectors

cmd/compile: reserve less memory for bitvectors

We only need 4 per block, not 7. That's almost 50% savings.

Update #77170

Change-Id: I6229a1137e7a03769e30bd4081078817de9a4f84
Reviewed-on: https://go-review.googlesource.com/c/go/+/736340
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>