isaspec: Give decode.c/h more descriptive names

Because these are being included across subdir boundaries, the name
"decode" is potentially pretty overloaded.  Instead, prefix them with
"isaspec_".  Also, since they're both weird includes now and not really
complete files in their own right, give them a descriptive suffix.

Acked-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20525>

src/compiler/isaspec/decode.py

@@ -192,7 +192,7 @@ const struct isa_bitset *${root.get_c_name()}[] = {
 };
 %endfor
 
-#include "decode.c"
+#include "isaspec_decode_impl.c"
 
 """
 
@@ -260,7 +260,7 @@ uint64_t_to_bitmask(uint64_t val)
     return mask;
 }
 
-#include "decode.h"
+#include "isaspec_decode_decl.h"
 
 #endif /* _${guard}_ */