For functions which check if a register pair is used, change them from
is_gp_pair to is_pair, since they do not care if the pair of register is
a gp pair or a fp pair.

Bug: v8:9909
Change-Id: I262108caadd616c64019b39ebbf6972a548112df
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1974833Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65526}