[dev.regabi] cmd/compile: use byte for CallExpr.Use

Reduce 16 byte for CallExpr, from 184 to 168 on 64-bit archs.

Passes toolstash -cmp.

Change-Id: I59c7609ccd03e8b4a7df8d2c30de8022ae312cee
Reviewed-on: https://go-review.googlesource.com/c/go/+/283732
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: cuonglm

src/cmd/compile/internal/ir/expr.go

@@ -145,7 +145,7 @@ func (n *BinaryExpr) SetOp(op Op) {
 }
 
 // A CallUse records how the result of the call is used:
-type CallUse int
+type CallUse byte
 
 const (
 	_ CallUse = iota