We can only replace phi(X, undef) with X, if X is known not to be
poison. Otherwise, the result may be more poisonous on the undef branch.

Fixes https://github.com/llvm/llvm-project/issues/68683.

When threading operations over phis, we need to adjust the context
instruction to the terminator of the incoming block. This was
handled when threading icmps, but not when threading binops.

Fixes https://github.com/llvm/llvm-project/issues/61312.

D128820 stopped creating div/rem constant expressions by default;
this patch removes support for them entirely.

The getUDiv(), getExactUDiv(), getSDiv(), getExactSDiv(), getURem()
and getSRem() on ConstantExpr are removed, and ConstantExpr::get()
now only accepts binary operators for which
ConstantExpr::isSupportedBinOp() returns true. Uses of these methods
may be replaced either by corresponding IRBuilder methods, or
ConstantFoldBinaryOpOperands (if a constant result is required).

On the C API side, LLVMConstUDiv, LLVMConstExactUDiv, LLVMConstSDiv,
LLVMConstExactSDiv, LLVMConstURem and LLVMConstSRem are removed and
corresponding LLVMBuild methods should be used.

Importantly, this also means that constant expressions can no longer
trap! This patch still keeps the canTrap() method to minimize diff --
I plan to drop it in a separate NFC patch.

Differential Revision: https://reviews.llvm.org/D129148

Unfortunately, it's not just constant expressions that can trap,
we might also have a trapping constant expression nested inside
a constant aggregate.

Perform the check during phi folding on Constant rather than
ConstantExpr, and extend the Constant::mayTrap() implementation
to also recursive into ConstantAggregates, not just ConstantExprs.

Fixes https://github.com/llvm/llvm-project/issues/49839.

This is a variant involving an aggregate constant, which was not
covered by the previous patch.

The only interesting test change is in @PR31262, where the following
fold is now performed, while it previously was not:
https://alive2.llvm.org/ce/z/a5Qmr6

llvm/test/Transforms/InstSimplify/ConstProp/gep.ll has not been
updated, because there is a tradeoff between folding and inrange
preservation there that we may want to discuss.

Updates have been performed using:
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34

Folding this case would result in the constant expression being
executed unconditionally, which may introduce a new trap.

Fixes https://github.com/llvm/llvm-project/issues/49839.