nir-to-tgsi: fix swizzle for double immediate

Use ureg_DECL_immediate_f64 to declare double immediate,
otherwise the swizzle for a double might not be in valid form,
ie. must be either .xy or .zw

Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <!16375>
154 jobs for !16375 with fix-ntt-double-imm in 26 minutes and 44 seconds (queued for 17 seconds)
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