- Detect cases where LHS & RHS values will not cause overflow
(when the Hi halfs are zero).

A generic alternative to #166564 - make the assumption that expanding
integer comparisons will be expensive if they are larger than the largest
legal type so avoid sinking if they are also used in the current BB + any phis.

Fixes #166534

Identified with modernize-use-equals-default.

Since new select/phi instructions may construct loops, the expression
tree to be simplified may still be incomplete (i.e., it may contain
select with dummy values or phi without incoming values). This patch
removes the call to simplifyInstruction for now, as it doesn't break
existing tests.

Original PR: https://reviews.llvm.org/D36073
Fix the crash reported in
https://github.com/llvm/llvm-project/pull/163453#issuecomment-3429922732.

This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]].  Note
that this patch adjusts the placement of [[maybe_unused]] to comply
with the C++17 language.

If we have integers larger than 64-bit we need to explicitly sign extend
them, otherwise we will get wrong zero extended values.

We match uadd's behavior here.

Codegen comparison: https://godbolt.org/z/x8j4EhGno

When deciding to sink address instructions into their uses, we check if
it is profitable to do so. The profitability check is based on the types
of uses of this address instruction -- if there are users which are not
memory instructions, then do not fold.

However, this profitability check wasn't considering target intrinsics,
which may be loads / stores.

This adds some logic to handle target memory intrinsics.

This is a reland of https://github.com/llvm/llvm-project/pull/158460

Test failures are gone once I undo the changes in codegenprepare.

Reverts llvm/llvm-project#158460 due to buildbot failures