[regalloc] Do not spill uses that are constrained to move to register in backwards spilling heuristics

For uses that are moved to registers, they are not beneficial for backwards spilling as it will introduce memory loads from stack to register.

Bug: chromium:1066869, chromium:1063831
Change-Id: I562d22336b6607a8f7286fc65dbf5b95a941a130
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2172052Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Yolanda Chen <yolanda.chen@intel.com>
Cr-Commit-Position: refs/heads/master@{#67765}