glsl: Fix lack of i2u in lower_ubo_reference.

glsl: Fix lack of i2u in lower_ubo_reference.

ir_binop_ubo_load takes unsigned integer operands.  However, the array
index used to compute these offsets may be a signed integer.  (For
example, see Piglit's spec/glsl-1.40/uniform_buffer/fs-bvec-array).

For some reason, we were missing an ir_binop_i2u cast, and ir_validator
was failing to catch that.

Without this change, ir_builder's type inference code broke for me when
writing a new optimization pass.

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