When dst is a fp pair, we set both low and high fp regs. Later when we
look at set regs to determine which registers to load into, we examine
both low and high fp. This is wrong - we only need to look at the low
fp, since Fill will load into the correct fp pairs. The bug was
triggered because we were examining into junk values in register_loads
indexed by the high fp.

Fixed: v8:10307
Change-Id: I6cbc212a969090818a5da0fe3dab36a418c23d04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2091632Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66632}