Implement ir_if (for if-statments) and conversion from AST

Implement ir_if (for if-statments) and conversion from AST

The following tests now pass:

    glslparsertest/shaders/if1.frag
    glslparsertest/shaders/if2.frag

The following tests that used to pass now fail.  It appears that most
of these fail because ast_nequal and ast_equal are not converted to HIR.

    shaders/glsl-unused-varying.frag
    shaders/glsl-fs-sqrt-branch.frag