More warnings fixed

Fixed warnings related to type conversions leading to potential loss of precision.

BUG=18368388

Change-Id: I71a7941df4bcf991f04818060780d4d395e335a9
Reviewed-on: https://swiftshader-review.googlesource.com/1393
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Shader/Shader.cpp b/src/Shader/Shader.cpp
index da0a84f..fc98eb5 100644
--- a/src/Shader/Shader.cpp
+++ b/src/Shader/Shader.cpp
@@ -1327,7 +1327,7 @@
 		return serialID;
 	}
 
-	int Shader::getLength() const
+	size_t Shader::getLength() const
 	{
 		return instruction.size();
 	}