Function prologues created slots for callee-saved registers twice on all platforms.
This didn't affect JS because it doesn't use callee-save, but would probably have
badly broken exceptions raised in Wasm code because Isolate::UnwindAndFindHandler
was restoring registers and SP incorrectly. It also broke the in-progress CL for
on-stack multiple returns.

No tests included with this fix, because currently it is almost impossible to test
directly (according to mstarzinger). But it will be tested indirectly via the upcoming
multi-return support.

Change-Id: If763cafc03de0a912eca48d5e25e8edfc4552b24
Reviewed-on: https://chromium-review.googlesource.com/758374Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49233}