glsl: Cast ir_call parameters to ir_rvalue, not ir_instruction.

glsl: Cast ir_call parameters to ir_rvalue, not ir_instruction.

A function call's parameters are always rvalues.  ir_rvalue may not
always be a subclass of ir_instruction in the future, so we should use
the right one.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>