glsl: Constant-fold built-in functions before outputting IR

glsl: Constant-fold built-in functions before outputting IR

Rearranged the logic for converting the ast for a function call to
hir, so that we constant fold before emitting any IR.  Previously we
would emit some IR, and then only later detect whether we could
constant fold.  The unnecessary IR would usually get cleaned up by a
later optimization step, however in the case of a builtin function
being used to compute an array size, it was causing an assertion.

Fixes Piglit test array-size-constant-relational.vert.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38625