ValueMapper: Remove old FIXMEs; almost NFC

Remove a few old FIXMEs from the original commit of the Metadata/Value
split in r223802.  These are commented out assertions to the effect that
calls between mapValue and mapMetadata never return nullptr.

(The only behaviour change is that Mapper::mapSimpleMetadata memoizes
the nullptr return.)

When I originally rewrote the mapping code, I thought we could be
stricter in the new metadata hierarchy and never return nullptr when
RF_NullMapMissingGlobalValues was off.  It's still not entirely clear to
me why these assertions failed (a few months ago, I had a theory that I
forgot to write down, but that's helping no one).

Understood or not, I no longer see how these commented-out assertions
would be useful.  I'm relegating them to the annals of source control
before making significant changes to ValueMapper.cpp.

llvm-svn: 265282