[SDAG] Introduce inbounds flag for ISD::PTRADD (#162477)

This patch introduces SDNodeFlags::InBounds, to show that an ISD::PTRADD SDNode
implements an inbounds getelementptr operation (i.e., the pointer operand is in
bounds wrt. an allocated object it is based on, and the arithmetic does not
change that). The flag is set in the DAG construction when lowering inbounds
GEPs.

Inbounds information is useful in the ISel when selecting memory instructions
that perform address computations whose intermediate steps must be in the same
memory region as the final result. Follow-up patches to propagate the flag in
DAGCombines and to use it when lowering AMDGPU's flat memory instructions,
where the immediate offset must not affect the memory aperture of the address
(similar to this GISel patch: #153001), are planned.

This mirrors #150900, which has introduced a similar flag in GlobalISel.

This patch supersedes #131862, which previously attempted to introduce an
SDNodeFlags::InBounds flag. The difference between this PR and #131862 is that
there is now an ISD::PTRADD opcode (PR #140017) and the InBounds flag is only
defined to apply to ISD::PTRADD DAG nodes. It is therefore unambiguous that
in-bounds-ness refers to a memory object into which the left operand of the
PTRADD node points (in contrast to #131862, where InBounds would have applied
to commutative ISD::ADD nodes, so that the semantics would be more difficult to
reason about).

For SWDEV-516125.

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