cmd/compile/internal/amd64: add SETccmem

Combine setcc and store of result into setcc that writes directly to memory.
Triggers 200+ times in go tool.

Fixes #21630

Change-Id: Iafa22607426f4120140c88fae4b9aecb46e0bba8
Reviewed-on: https://go-review.googlesource.com/67950
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/asm_test.go

@@ -1120,6 +1120,14 @@ var linuxAMD64Tests = []*asmTest{
 		`,
 		pos: []string{"\tMOVL\t[^X].*, X.*"},
 	},
+	{
+		fn: `
+		func $(x uint32) bool {
+			return x > 4
+		}
+		`,
+		pos: []string{"\tSETHI\t\\("},
+	},
 }
 
 var linux386Tests = []*asmTest{