cmd/5c: re-enable regopt()

        After CL 6185047, ./all.bash passed.

benchmark                       old ns/op    new ns/op    delta
BenchmarkAppend                      5558         4894  -11.95%
BenchmarkAppendSpecialCase           5242         4572  -12.78%
BenchmarkSelectUncontended           3719         2821  -24.15%
BenchmarkSelectContended             3776         2832  -25.00%
BenchmarkSelectNonblock              1030         1089   +5.73%
BenchmarkChanUncontended              530          422  -20.38%
BenchmarkChanContended                534          444  -16.85%
BenchmarkChanSync                    1613         1492   -7.50%
BenchmarkChanProdCons0               1520         1351  -11.12%
BenchmarkChanProdCons10               785          668  -14.90%
BenchmarkChanProdCons100              564          473  -16.13%
BenchmarkChanProdConsWork0          11205        10337   -7.75%
BenchmarkChanProdConsWork10          9806         9567   -2.44%
BenchmarkChanProdConsWork100         9413         9398   -0.16%
BenchmarkChanCreation               11687         8715  -25.43%
BenchmarkChanSem                      553          453  -18.08%
BenchmarkCallClosure                   22           22   +0.44%
BenchmarkCallClosure1                  28           28   +0.71%
BenchmarkCallClosure2                2224         1668  -25.00%
BenchmarkCallClosure3                2217         1629  -26.52%
BenchmarkCallClosure4                2240         1684  -24.82%
BenchmarkComplex128DivNormal          930          912   -1.94%
BenchmarkComplex128DivNisNaN          862          866   +0.46%
BenchmarkComplex128DivDisNaN          849          852   +0.35%
BenchmarkComplex128DivNisInf          556          583   +4.86%
BenchmarkComplex128DivDisInf          522          512   -1.92%
BenchmarkConvT2E                      175          159   -9.14%
BenchmarkConvT2EBig                  2418         1823  -24.61%
BenchmarkConvT2I                      545          549   +0.73%
BenchmarkConvI2E                       35           32   -9.58%
BenchmarkConvI2I                      404          391   -3.22%
BenchmarkAssertE2T                     75           62  -16.25%
BenchmarkAssertE2TBig                  76           63  -16.80%
BenchmarkAssertE2I                    427          409   -4.22%
BenchmarkAssertI2T                     82           66  -20.29%
BenchmarkAssertI2I                    430          416   -3.26%
BenchmarkAssertI2E                     36           32  -12.50%
BenchmarkAssertE2E                     35           35   +0.57%
BenchmarkFinalizer                   3224         2941   -8.78%
BenchmarkFinalizerRun              117392        84772  -27.79%
BenchmarkStackGrowth                 5267         5930  +12.59%
BenchmarkSyscall                      191          167  -12.57%
BenchmarkSyscallWork                 9918         7713  -22.23%
BenchmarkIfaceCmp100                 1645         1652   +0.43%
BenchmarkIfaceCmpNil100              1433         1440   +0.49%

R=dave, rsc
CC=golang-dev
https://golang.org/cl/6202070

src/cmd/5c/reg.c

@@ -66,12 +66,6 @@ rcmp(const void *a1, const void *a2)
 void
 regopt(Prog *p)
 {
-	USED(p);
-	// TODO(kaib): optimizer disabled because it smashes R8 when running out of registers
-	// the disable is unconventionally here because the call is in common code shared by 5c/6c/8c
-	return;
-
-#ifdef	NOTDEF
 	Reg *r, *r1, *r2;
 	Prog *p1;
 	int i, z;
@@ -500,7 +494,6 @@ brk:
 		r1->link = freer;
 		freer = firstr;
 	}
-#endif
 }
 
 void