cmd/compile/internal/ssa: optimize ANDconst rule of loong64

Change-Id: I0e88f885ff17b4932c2f448dc3c577c0329a6658
Reviewed-on: https://go-review.googlesource.com/c/go/+/620976
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: abner chenc <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: limeidan

src/cmd/compile/internal/ssa/_gen/LOONG64.rules

@@ -730,6 +730,8 @@
 (MOVWUreg (MOVVconst [c])) => (MOVVconst [int64(uint32(c))])
 (MOVVreg  (MOVVconst [c])) => (MOVVconst [c])
 
+(MOVBUreg (ANDconst [c] x)) => (ANDconst [c&0xff] x)
+
 // constant comparisons
 (SGTconst [c] (MOVVconst [d])) && c>d => (MOVVconst [1])
 (SGTconst [c] (MOVVconst [d])) && c<=d => (MOVVconst [0])