{PopToRegister} will most likely find that the stack slot is already
holding a register (89% of cases on epic). Thus put the fast path for
this in the header, so it can be inlined.

Also, {GetUnusedRegister} will mostly find an unused register (95% on
epic). Hence, make sure that the code for spilling a register is not
inlined.

Drive-by: Avoid the call to {LoadToRegister} if we already checked
before if the slot is holding a register.

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: I13797fa5c12c5359f2578a4dbebb63aa50c00e60
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237144
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68280}