This prevents us from ending up with (zext (sext X)). The zext will
require an instruction on targets where zext isn't free like RISC-V.

When IndVarSimplify simplifies a rem of the induction variable to a cmp
and select, only the select currently receives the rem's source
location; this patch propagates it to the cmp as well.

Found using https://github.com/llvm/llvm-project/pull/107279.

`WidenIV::widenWithVariantUse` assumes that exactly one of the binop
operands is the IV to be widened. This miscompilation happens when it
tries to sign-extend the "NonIV" operand while the IV is zero-extended.
Closes https://github.com/llvm/llvm-project/issues/135182.

Proof: https://alive2.llvm.org/ce/z/NVXaeo

Relevant parts of ScalarEvolution's API accept a CmpPredicate instead of
a CmpInst::Predicate after 60dc450 (SCEV: migrate to CmpPredicate
(NFC)). After auditing the callers of these APIs, it was found that
IndVarSimplify was dropping samesign information. Fix this.

With the introduction of CmpPredicate in 51a895a (IR: introduce struct
with CmpInst::Predicate and samesign), PatternMatch is one of the first
key pieces of infrastructure that must be updated to match a CmpInst
respecting samesign information. Implement this change to Cmp-matchers.

This is a preparatory step in migrating the codebase over to
CmpPredicate. Since we no functional changes are desired at this stage,
we have chosen not to migrate CmpPredicate::operator==(CmpPredicate)
calls to use CmpPredicate::getMatching(), as that would have visible
impact on tests that are not yet written: instead, we call
CmpPredicate::operator==(Predicate), preserving the old behavior, while
also inserting a few FIXME comments for follow-ups.

This macros is always defined: either 0 or 1. The correct pattern is to
use #if.

Re-apply #110185 with more fixes for debug build with the ABI breaking
checks disabled.