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 <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: thanm

src/cmd/compile/internal/base/flag.go

@@ -184,6 +184,7 @@ func ParseFlags() {
 	Debug.SyncFrames = -1 // disable sync markers by default
 	Debug.ZeroCopy = 1
 	Debug.RangeFuncCheck = 1
+	Debug.MergeLocals = 1
 
 	Debug.Checkptr = -1 // so we can tell whether it is set explicitly