cmd/compile: minor doc improvements

These are left over from comments I failed to leave on CL 249463;
apparently I never hit "Reply".

Change-Id: Ia3f8a900703c347f8f98581ec1ac172c0f72cd9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/299589
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -24,7 +24,7 @@ func decomposeBuiltIn(f *Func) {
 	}
 
 	// Decompose other values
-	// Note: deadcode is false because we need to keep the original
+	// Note: Leave dead values because we need to keep the original
 	// values around so the name component resolution below can still work.
 	applyRewrite(f, rewriteBlockdec, rewriteValuedec, leaveDeadValues)
 	if f.Config.RegSize == 4 {

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

@@ -27,7 +27,7 @@ const (
 	removeDeadValues                 = true
 )
 
-// deadcode indicates that rewrite should try to remove any values that become dead.
+// deadcode indicates whether rewrite should try to remove any values that become dead.
 func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter, deadcode deadValueChoice) {
 	// repeat rewrites until we find no more rewrites
 	pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block

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

@@ -499,7 +499,7 @@ func (v *Value) removeable() bool {
 		return false
 	}
 	if v.Type.IsMemory() {
-		// All memory ops aren't needed here, but we do need
+		// We don't need to preserve all memory ops, but we do need
 		// to keep calls at least (because they might have
 		// synchronization operations we can't see).
 		return false