cmd/compile: join indexed byte loads into larger loads

Fixes #14920

Change-Id: I1535dc529779e26141d92d9e2b6ba7b016590c1a
Reviewed-on: https://go-review.googlesource.com/21005
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Ahmed Waheed <oneofone@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1260,3 +1260,22 @@
     (SHLQconst [40] (MOVBload [i+5] {s} p mem)))
     (SHLQconst [48] (MOVBload [i+6] {s} p mem)))
     (SHLQconst [56] (MOVBload [i+7] {s} p mem))) -> @x.Block (MOVQload [i] {s} p mem)
+
+(ORW              x:(MOVBloadidx1 [i]   {s} p idx mem)
+    (SHLWconst [8]  (MOVBloadidx1 [i+1] {s} p idx mem))) -> @x.Block (MOVWload [i] {s} (ADDQ <p.Type> p idx) mem)
+
+(ORL (ORL (ORL
+                  x:(MOVBloadidx1 [i]   {s} p idx mem)
+    (SHLLconst [8]  (MOVBloadidx1 [i+1] {s} p idx mem)))
+    (SHLLconst [16] (MOVBloadidx1 [i+2] {s} p idx mem)))
+    (SHLLconst [24] (MOVBloadidx1 [i+3] {s} p idx mem))) -> @x.Block (MOVLload [i] {s} (ADDQ <p.Type> p idx) mem)
+
+(ORQ (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ
+                  x:(MOVBloadidx1 [i]   {s} p idx mem)
+    (SHLQconst [8]  (MOVBloadidx1 [i+1] {s} p idx mem)))
+    (SHLQconst [16] (MOVBloadidx1 [i+2] {s} p idx mem)))
+    (SHLQconst [24] (MOVBloadidx1 [i+3] {s} p idx mem)))
+    (SHLQconst [32] (MOVBloadidx1 [i+4] {s} p idx mem)))
+    (SHLQconst [40] (MOVBloadidx1 [i+5] {s} p idx mem)))
+    (SHLQconst [48] (MOVBloadidx1 [i+6] {s} p idx mem)))
+    (SHLQconst [56] (MOVBloadidx1 [i+7] {s} p idx mem))) -> @x.Block (MOVQload [i] {s} (ADDQ <p.Type> p idx) mem)