cmd/compile/internal/base: enable stack slot merging by default

cmd/compile/internal/base: enable stack slot merging by default

Flag flip to enable stack slot merging by default when optimizing.
Please see the earlier CL for details on what this is doing.

Updates #62737.
Updates #65532.
Updates #65495.

Change-Id: I8e30d553e74ace43d418f883199721f05320d3d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/576681
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>