glsl/lower_instructions: Use float16 constants when appropriate

glsl/lower_instructions: Use float16 constants when appropriate

When lowering instructions that involve floating-point constants, pick
the appropriate type for the constant so that it will also work with
float16 parameters.

v2: Use float16_t constructor instead of helper function.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3929>