glsl: Combine AST-level and IR-level parameter mode checking loops.

glsl: Combine AST-level and IR-level parameter mode checking loops.

generate_call() and ast_function_expression::hir() both tried to verify
that 'out' and 'inout' parameters used l-values.  Irritatingly, it
turned out that this was not redundant; both checks caught -some- cases.

This patch combines the two into a single "complete" function that does
all the parameter mode checking.  It also adds a comment clarifying why
AST-level checking is necessary in the first place.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>