[PATCH] [llgo] Optimize comparisons to empty string.

[Andrew Wilkins]

LGTM.

================
Comment at: irgen/strings.go:29-31
@@ +28,5 @@
+	len := fr.builder.CreateExtractValue(v, 1, "")
+	zero := llvm.ConstNull(len.Type())
+	result := fr.builder.CreateICmp(llvm.IntEQ, len, zero, "")
+	result = fr.builder.CreateZExt(result, llvm.Int8Type(), "")
+	return newValue(result, types.Typ[types.Bool])
----------------
result := fr.builder.CreateIsNull(len, "")
?

http://reviews.llvm.org/D6569