cmd/compile: place combined loads at the location of the last byte load

We need to make sure all the bounds checks pass before issuing
a load which combines several others.  We do this by issuing the
combined load at the last load's block, where "last" = closest to
the leaf of the dominator tree.

Fixes #15002

Change-Id: I7358116db1e039a072c12c0a73d861f3815d72af
Reviewed-on: https://go-review.googlesource.com/21246
Reviewed-by: Todd Neal <todd@tneal.org>

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

@@ -261,3 +261,53 @@ func duff(size int64) (int64, int64) {
 	}
 	return off, adj
 }
+
+// mergePoint finds a block among a's blocks which dominates b and is itself
+// dominated by all of a's blocks. Returns nil if it can't find one.
+// Might return nil even if one does exist.
+func mergePoint(b *Block, a ...*Value) *Block {
+	// Walk backward from b looking for one of the a's blocks.
+
+	// Max distance
+	d := 100
+
+	for d > 0 {
+		for _, x := range a {
+			if b == x.Block {
+				goto found
+			}
+		}
+		if len(b.Preds) > 1 {
+			// Don't know which way to go back. Abort.
+			return nil
+		}
+		b = b.Preds[0]
+		d--
+	}
+	return nil // too far away
+found:
+	// At this point, r is the first value in a that we find by walking backwards.
+	// if we return anything, r will be it.
+	r := b
+
+	// Keep going, counting the other a's that we find. They must all dominate r.
+	na := 0
+	for d > 0 {
+		for _, x := range a {
+			if b == x.Block {
+				na++
+			}
+		}
+		if na == len(a) {
+			// Found all of a in a backwards walk. We can return r.
+			return r
+		}
+		if len(b.Preds) > 1 {
+			return nil
+		}
+		b = b.Preds[0]
+		d--
+
+	}
+	return nil // too far away
+}