intel/fs: Fix handling of W, UW, and HF constants in combine_constants

intel/fs: Fix handling of W, UW, and HF constants in combine_constants

Sources that are already W, UW, or HF can be represented as those types
by definition. Pass them through. Previously an HF source on a MAD would
have been marked as !can_promote. I'm pretty sure this means it would
get moved out to a register, but I did not verify this.

For ADD3, a constant source could be D or UD. In this case, the value
must be tested to determine whether it can be represented as W or
UW. The patterns in opt_algebraic won't generate an ADD3 with constant
source, so this problem cannot occur yet.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23262>