glsl/ir: add subroutine lowering pass (v2.3)

glsl/ir: add subroutine lowering pass (v2.3)

This lowers the enhanced ir_call using the lookaside table
of subroutines into an if ladder. This initially was done
at the AST level but it caused some ordering issues so a separate
pass was required.

v2: clone return value derefs.
v2.1: update for subroutine->int convert.
v2.2: add a clone for the array index

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Signed-off-by: Dave Airlie <airlied@redhat.com>