cmd/compile: fix offset calculation error in memcombine

Fixes #73812

Change-Id: If7a6e103ae9e1442a2cf4a3c6b1270b6a1887196
Reviewed-on: https://go-review.googlesource.com/c/go/+/675175
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ssa/memcombine.go

@@ -581,7 +581,7 @@ func combineStores(root *Value) {
 			mask := int64(1)<<(8*a[i].size) - 1
 			s := 8 * (a[i].offset - a[0].offset)
 			if root.Block.Func.Config.BigEndian {
-				s = aTotalSize*8 - a[i].size - s
+				s = (aTotalSize-a[i].size)*8 - s
 			}
 			c |= (a[i].store.Args[1].AuxInt & mask) << s
 		}

test/codegen/issue72832.go

@@ -35,7 +35,16 @@ type tile3 struct {
 
 func store_shifted(t *tile3, x uint32) {
 	// amd64:`MOVL`
+	// ppc64:`MOVHBR`
 	t.a = uint8(x)
 	t.b = uint8(x >> 8)
 	t.c = uint16(x >> 16)
 }
+
+func store_const(t *tile3) {
+	// 0x00030201
+	// amd64:`MOVL\s\$197121`
+	// 0x01020003
+	// ppc64:`MOVD\s\$16908291`
+	t.a, t.b, t.c = 1, 2, 3
+}