glsl: Expand fp16 to float before constant expression evaluation

glsl: Expand fp16 to float before constant expression evaluation

This way the generated constant folding code doesn't need to
understand fp16.  All operations have to be expanded to full float for
evaulation on the CPU, so we might as well do it up front.  As far as
GLSL is concerned, fp16 isn't a separate type from float, so
everything we're supposed to support for float we need to do for fp16.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3929>