[ValueTracking] Make poison propagation more aggressive

Description

[ValueTracking] Make poison propagation more aggressive

Summary:
Motivation: fix PR31181 without regression (the actual fix is still in
progress). However, the actual content of PR31181 is not relevant
here.

This change makes poison propagation more aggressive in the following
cases:

  1. poision * Val == poison, for any Val. In particular, this changes existing intentional and documented behavior in these two cases: a. Val is 0 b. Val is 2^k * N
  2. poison << Val == poison, for any Val
  3. getelementptr is poison if any input is poison

I think all of these are justified (and are axiomatically true in the
new poison / undef model):

1a: we need poison * 0 to be poison to allow transforms like these:

A * (B + C) ==> A * B + A * C

If poison * 0 were 0 then the above transform could not be allowed
since e.g. we could have A = poison, B = 1, C = -1, making the LHS

poison * (1 + -1) = poison * 0 = 0

and the RHS

poison * 1 + poison * -1 = poison + poison = poison

1b: we need e.g. poison * 4 to be poison since we want to allow

A * 4 ==> A + A + A + A

If poison * 4 were a value with all of their bits poison except the
last four; then we'd not be able to do this transform since then if A
were poison the LHS would only be "partially" poison while the RHS
would be "full" poison.

2: Same reasoning as (1b), we'd like have the following kinds
transforms be legal:

A << 1 ==> A + A

Reviewers: majnemer, efriedma

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D30185

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