cmd/compile: omit racefuncentry/exit when they are not needed

When compiling with -race, we insert calls to racefuncentry,
into every function. Add a rule that removes them in leaf functions,
without instrumented loads/stores.
Shaves ~30kb from "-race" version of go tool:

file difference:
go_old 15626192
go_new 15597520 [-28672 bytes]

section differences:
global text (code) = -24513 bytes (-0.358598%)
read-only data = -5849 bytes (-0.167064%)
Total difference -30362 bytes (-0.097928%)

Fixes #24662

Change-Id: Ia63bf1827f4cf2c25e3e28dcd097c150994ade0a
Reviewed-on: https://go-review.googlesource.com/121235
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -38,6 +38,7 @@ type Config struct {
 	nacl           bool          // GOOS=nacl
 	use387         bool          // GO386=387
 	SoftFloat      bool          //
+	Race           bool          // race detector enabled
 	NeedsFpScratch bool          // No direct move between GP and FP register sets
 	BigEndian      bool          //
 }