SpirvShader: Implement GLSLstd450Log2

Bug: b/126873455
Tests: dEQP-VK.glsl.builtin.precision.log2.*
Change-Id: I1a6a4e1fbf41ccd7aacf2fa7fbe8b8e7c6d28f3b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28443
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
diff --git a/src/Reactor/Reactor.hpp b/src/Reactor/Reactor.hpp
index 86e3b70..4e74812 100644
--- a/src/Reactor/Reactor.hpp
+++ b/src/Reactor/Reactor.hpp
@@ -2228,6 +2228,7 @@
 	RValue<Float4> Exp(RValue<Float4> x);
 	RValue<Float4> Log(RValue<Float4> x);
 	RValue<Float4> Exp2(RValue<Float4> x);
+	RValue<Float4> Log2(RValue<Float4> x);
 
 	template<class T>
 	class Pointer : public LValue<Pointer<T>>