glsl: Add an ir_expression triop constructor with type inference.

glsl: Add an ir_expression triop constructor with type inference.

We already have ir_expression constructors for unary and binary
operations, which automatically infer the type based on the opcode and
operand types.

These are convenient and also required for ir_builder support.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>