.. to increase coverage of concurrent inlining, at least in this stress
mode. The common pattern in mjsunit tests is to call
`%OptimizeFunctionOnNextCall(f)` for interesting function `f`. This
explicitly triggers non-concurrent compilation, significantly decreasing
relevant coverage of concurrent inlining.

This CL recovers coverage by spawning an additional concurrent compile
job when 1. --stress-concurrent-inlining is enabled, and 2. the
requested compile mode is non-concurrent. The result of these
additional jobs is discarded.

Drive-by: Fix two simple uncovered issues.

Bug: v8:7790,v8:11513,v8:11648
Change-Id: If1e8ca5ba737e3cecdec9e15e4a86b28fe9fb2de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2824440
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73967}