cmd/compile: fix build

Run live vars test only on ssa builds.
We can't just drop KeepAlive ops during regalloc.  We need
to replace them with copies.

Change-Id: Ib4b3b1381415db88fdc2165fc0a9541b73ad9759
Reviewed-on: https://go-review.googlesource.com/23225
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -948,13 +948,14 @@ func (s *regAllocState) regalloc(f *Func) {
 				if vi.spillUsed {
 					// Use the spill location.
 					v.SetArg(0, vi.spill)
-					b.Values = append(b.Values, v)
 				} else {
 					// No need to keep unspilled values live.
 					// These are typically rematerializeable constants like nil,
 					// or values of a variable that were modified since the last call.
-					v.Args[0].Uses--
+					v.Op = OpCopy
+					v.SetArgs1(v.Args[1])
 				}
+				b.Values = append(b.Values, v)
 				continue
 			}
 			regspec := opcodeTable[v.Op].reg

test/fixedbugs/issue15277.go

@@ -3,6 +3,7 @@
 // Copyright 2016 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+// +build amd64
 
 package main