Revert "Unify implementation of our two different flavours of -Wtautological-compare."

> Unify implementation of our two different flavours of -Wtautological-compare.
>
> In so doing, fix a handful of remaining bugs where we would report false
> positives or false negatives if we promote a signed value to an unsigned type
> for the comparison.

This caused a new warning in Chromium:

../../base/trace_event/trace_log.cc:1545:29: error: comparison of constant 64
with expression of type 'unsigned int' is always true
[-Werror,-Wtautological-constant-out-of-range-compare]
  DCHECK(handle.event_index < TraceBufferChunk::kTraceBufferChunkSize);
         ~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The 'unsigned int' is really a 6-bit bitfield, which is why it's always
less than 64.

I thought we didn't use to warn (with out-of-range-compare) when comparing
against the boundaries of a type?

llvm-svn: 320162