This PR removes tests with `br i1 undef` under
`llvm/tests/Transforms/Loop*, Lower*`.

This extends the work from 7755c26 to all of the different backend
taken count kinds that we print for the scev analysis printer.  As
before, the goal is to cut down on confusion as i4 -1 is a very
different (unsigned) value from i32 -1.

When printing the result of SCEV's analysis, we can avoid printing
the predicated backedge taken count and the predicates if the predicates
are empty and no new information is provided.  This helps to reduce the
verbosity of the output.

When printing the result of the analysis, i8 -1 and i64 -1 are quite
different in terms of analysis quality.  In a recent conversion with
a new contributor, we ran into exactly this confusion.

Adding the type for constant scevs more globally seems worthwhile, but
introduces a much larger test diff.  I'm splitting this off first since
it addresses the immediate need, and then going to do some further
changes to clarify a few related bits of analysis result output.

When a SCEVCallbackVH is RAUWed, we currently do a def-use walk and
remove dependent instructions from the ValueExprMap. However, unlike
SCEVs usual invalidation, this does not forget memoized values.

The end result is that we might end up removing a SCEVUnknown from the
map, while that expression still has users. Due to that, we may later
fail to invalide those expressions. In particular, invalidation of loop
dispositions only does something if there is an expression for the
value, which would not be the case here.

Fix this by using the standard forgetValue() API, instead of rolling a
custom variant.

Fixes https://github.com/llvm/llvm-project/issues/68285.

This adds code to the loop rotation transformation to ensure that the
computed block execution counts for the loop bodies are the same before
and after the transformation. This isn't always true in practice, but I
believe this is because of numeric inaccuracies in the BlockFrequency
computation.

The invariants this is modeled on and heuristic choice of 0-trip loop
amount is explained in a lenghty comment in the new
`updateBranchWeights()` function.

Differential Revision: https://reviews.llvm.org/D157462

Set phi inputs to poison whenever we find a dead edge (either
during initial worklist population or the main InstCombine run),
instead of only doing this for successors of dead blocks.

This means that the phi operand is set to poison even if for
critical edges without an intermediate block.

There are quite a few test changes, because the pattern is fairly
common in vectorizer output, for cases where we know the vectorized
loop will be entered.

This reverts commit 706e8110573c83f140a63b40803d6370c86c1414.