glsl/lower_if: move and rename found_control_flow
glsl/lower_if: move and rename found_control_flow

I'll want to update more variables in check_control_flow, so using
the visitor is convenient.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler