cmd/compile: avoid calling unnecessary Sym format routine

Minor cleanup only.

No reason to go through String() when it is
just as easy to do a direct string comparison.

Eliminates a surprising number of allocations.

name       old alloc/op    new alloc/op    delta
Template      40.9MB ± 0%     40.9MB ± 0%    ~     (p=0.190 n=10+10)
Unicode       30.3MB ± 0%     30.3MB ± 0%    ~     (p=0.218 n=10+10)
GoTypes        116MB ± 0%      116MB ± 0%  -0.09%  (p=0.000 n=10+10)
SSA            871MB ± 0%      869MB ± 0%  -0.14%  (p=0.000 n=10+9)
Flate         26.2MB ± 0%     26.2MB ± 0%  -0.15%  (p=0.002 n=10+10)
GoParser      32.5MB ± 0%     32.5MB ± 0%    ~     (p=0.165 n=10+10)
Reflect       80.5MB ± 0%     80.4MB ± 0%  -0.12%  (p=0.003 n=9+10)
Tar           27.3MB ± 0%     27.3MB ± 0%  -0.13%  (p=0.008 n=10+9)
XML           43.1MB ± 0%     43.1MB ± 0%    ~     (p=0.218 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        402k ± 1%       400k ± 1%  -0.64%  (p=0.002 n=10+10)
Unicode         322k ± 1%       321k ± 1%    ~     (p=0.075 n=10+10)
GoTypes        1.19M ± 0%      1.18M ± 0%  -0.90%  (p=0.000 n=10+10)
SSA            7.94M ± 0%      7.81M ± 0%  -1.66%  (p=0.000 n=10+9)
Flate           246k ± 0%       242k ± 1%  -1.42%  (p=0.000 n=10+10)
GoParser        325k ± 1%       323k ± 1%  -0.84%  (p=0.000 n=10+10)
Reflect        1.02M ± 0%      1.01M ± 0%  -0.99%  (p=0.000 n=10+10)
Tar             259k ± 0%       257k ± 1%  -0.72%  (p=0.009 n=10+10)
XML             406k ± 1%       403k ± 1%  -0.69%  (p=0.001 n=10+10)

Change-Id: Ia129a4cd272027d627e1f3b27e9f07f93e3aa27e
Reviewed-on: https://go-review.googlesource.com/38230Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -3205,7 +3205,7 @@ func (s *state) canSSA(n *Node) bool {
 			return false
 		}
 	}
-	if n.Class == PPARAM && n.String() == ".this" {
+	if n.Class == PPARAM && n.Sym != nil && n.Sym.Name == ".this" {
 		// wrappers generated by genwrapper need to update
 		// the .this pointer in place.
 		// TODO: treat as a PPARMOUT?