glsl/ir_builder: Add helpers for making if-statements

glsl/ir_builder: Add helpers for making if-statements

Add two overloaded variants of
    ir_if *if_tree()

The new functions allow one to chain together if-trees within a single C++
expression that resembles a real if-statement.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>