cmd/compile: label LoadReg with line number of the use

A tentative fix of #16380. It adds "line" everywhere...

This also reduces binary size slightly (cmd/go on ARM as an example):

			before		after
total binary size	8068097		8018945 (-0.6%)
.gopclntab		1195341		1179929 (-1.3%)
.debug_line		 689692		 652017 (-5.5%)

Change-Id: Ibda657c6999783c5bac180cbbba487006dbf0ed7
Reviewed-on: https://go-review.googlesource.com/25082Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -111,7 +111,7 @@ func dse(f *Func) {
 				if sz > 0x7fffffff { // work around sparseMap's int32 value type
 					sz = 0x7fffffff
 				}
-				shadowed.set(v.Args[0].ID, int32(sz))
+				shadowed.set(v.Args[0].ID, int32(sz), 0)
 			}
 		}
 		// walk to previous store

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

@@ -10,6 +10,7 @@ package ssa
 type sparseEntry struct {
 	key ID
 	val int32
+	aux int32
 }
 
 type sparseMap struct {
@@ -42,13 +43,14 @@ func (s *sparseMap) get(k ID) int32 {
 	return -1
 }
 
-func (s *sparseMap) set(k ID, v int32) {
+func (s *sparseMap) set(k ID, v, a int32) {
 	i := s.sparse[k]
 	if i < int32(len(s.dense)) && s.dense[i].key == k {
 		s.dense[i].val = v
+		s.dense[i].aux = a
 		return
 	}
-	s.dense = append(s.dense, sparseEntry{k, v})
+	s.dense = append(s.dense, sparseEntry{k, v, a})
 	s.sparse[k] = int32(len(s.dense)) - 1
 }
 
@@ -62,7 +64,7 @@ func (s *sparseMap) setBit(k ID, v uint) {
 		s.dense[i].val |= 1 << v
 		return
 	}
-	s.dense = append(s.dense, sparseEntry{k, 1 << v})
+	s.dense = append(s.dense, sparseEntry{k, 1 << v, 0})
 	s.sparse[k] = int32(len(s.dense)) - 1
 }