[release/10.0] Fix bug in LowerCallMemmove

[Copilot]

main PR #123907

Description

LowerCallMemmove handled CORINFO_HELP_MEMCPY by setting BlkOpKindUnroll on the newly constructed STORE_BLK, but then skipped re-lowering srcBlk/storeBlk entirely. This left the address nodes in an unlowered state, allowing them to become contained — violating the contract that genCodeForMemmove (and genCodeForCpBlkUnroll) rely on.

Fix: always use BlkOpKindUnrollMemmove for both memmove and memcpy paths. The nodes are hand-constructed already in their final lowered form, so genCodeForMemmove handles them correctly without a separate lowering pass.

• assert(!dstAddr->isContained()) / assert(!srcAddr->isContained()) — enforces the invariant genCodeForMemmove relies on, and catches future regressions.
• Remove conditional BlkOpKindUnroll — use BlkOpKindUnrollMemmove unconditionally; avoids the need to re-lower nodes created by this function.
• Update comment — replaces misleading // TODO: This skips lowering srcBlk and storeBlk. with an accurate explanation.

Customer Impact

Unsafe.CopyBlockUnaligned / cpblk with a size that becomes constant in a late JIT phase can produce incorrect code on affected builds.

Regression

• Yes
• No

Present since CORINFO_HELP_MEMCPY handling was added to LowerCallMemmove.

Testing

Covered by the existing memmove/memcpy unrolling test suite. The original fix in main was validated with no jit-diffs (see PR #123907).

Risk

Low. Single-function change, logic simplification only — removes a conditional branch and adds debug-only assertions. The CORINFO_HELP_MEMCPY unroll path is rarely exercised (requires constant-length cpblk resolved in a late phase). BlkOpKindUnrollMemmove is already the well-tested code path for the memmove case.

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