glsl: Initialize ir_to_mesa_visitor members in constructor.

glsl: Initialize ir_to_mesa_visitor members in constructor.

Fix defects reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member ctx is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member prog is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member shader_program is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member options is not initialized in this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7071>