glsl: correct bitcast-helpers

Without this, we'll incorrectly round off huge values to the nearest
representable double instead of keeping it at the exact value  as
we're supposed to.

Found by inspecting compiler-warnings.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Fixes: 85faf508 ("glsl: Add 64-bit integer support for constant expressions")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 88f909eb)