Analyze the shader for define instructions.

This eliminates the need to specify if shaders can contain defined
constant values in the front-end using a global variable.

Change-Id: If7802a2743c0afa650a2631cd7945c8b3d7cf645
Reviewed-on: https://swiftshader-review.googlesource.com/3152
Reviewed-by: Greg Hartman <ghartman@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>