[dev.ssa] cmd/compile: don't move OpSP and OpSB values

regalloc expects to find all OpSP and OpSB values
in the entry block.

There is no value to moving them; don't.

Change-Id: I775198f03ce7420348721ffc5e7d2bab065465b1
Reviewed-on: https://go-review.googlesource.com/13266Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -101,7 +101,8 @@ func regalloc(f *Func) {
 
 	var oldSched []*Value
 
-	// Hack to find sp and sb Values and assign them a register. (TODO: make not so hacky)
+	// Hack to find sp and sb Values and assign them a register.
+	// TODO: make not so hacky; update the tighten pass when this is done
 	var sp, sb *Value
 	for _, v := range f.Entry.Values {
 		switch v.Op {

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

@@ -57,6 +57,13 @@ func tighten(f *Func) {
 				if v.Op == OpPhi {
 					continue
 				}
+				if v.Op == OpSB || v.Op == OpSP {
+					// regalloc expects OpSP and OpSB values to be in the entry block,
+					// so don't move them.
+					// TODO: Handle this more gracefully in regalloc and
+					// remove this restriction.
+					continue
+				}
 				if uses[v.ID] == 1 && !phi[v.ID] && home[v.ID] != b && len(v.Args) < 2 {
 					// v is used in exactly one block, and it is not b.
 					// Furthermore, it takes at most one input,