Quick compiler, out of registers fix

It turns out that the register pool sanity checker was not
working as expected, leaving some inconsistencies unreported.
This could result in "out of registers" failures, as well
as other more subtle problems.

This CL fixes the sanity checker, adds a lot more check and cleans
up the previously undetected episodes of insanity.

Cherry-pick of internal change 468162

Change-Id: Id2da97e99105a4c272c5fd256205a94b904ecea8