Revert "Fix comments in instr_time.h and remove an unneeded cast to int64"

 Revert "Fix comments in instr_time.h and remove an unneeded cast to int64"
 
 This reverts commit 3dcb09de7b. Tom Lane pointed out that it broke the
 abstraction provided by the macros. The callers should not need to
 know what the internal type is.
 
 This commit is an exact revert, the next commit will fix the comments
 on the macros that incorrectly claim that they return uint64.
 
 Discussion: https://www.postgresql.org/message-id/ZrHkv3MAQfwNSmTG@ip-10-97-1-34.eu-west-3.compute.internal