[LowerMemIntrinsics] Factor control flow generation out of the memcpy lowering (#169039)

So far, memcpy with known size, memcpy with unknown size, memmove with known
size, and memmove with unknown size have individual optimized loop lowering
implementations, while memset and memset.pattern use an unoptimized loop
lowering. This patch extracts the parts of the memcpy lowerings (for known and
unknown sizes) that generate the control flow for the loop expansion into an
`insertLoopExpansion` function. The `createMemCpyLoop(Unk|K)nownSize` functions
then only collect the necessary arguments for `insertLoopExpansion`, call it,
and fill the generated loop basic blocks.

The immediate benefit of this is that logic from the two memcpy lowerings is
deduplicated. Moreover, it enables follow-up patches that will use
`insertLoopExpansion` to optimize the memset and memset.pattern implementations
similarly to memcpy, since they can use the exact same control flow patterns.

The test changes are due to more consistent and useful basic block names in the
loop expansion and an improvement in basic block ordering: previously, the
basic block that determines if the residual loop is executed would be put at
the end of the function, now it is put before the residual loop body.
Otherwise, the generated code should be equivalent.

This patch doesn't affect memmove; deduplicating its logic would also be nice,
but to extract all CF generation from the memmove lowering,
`insertLoopExpansion` would need to be able to also create code that iterates
backwards over the argument buffers. That would make `insertLoopExpansion` a
lot more complex for a code path that's only used for memmove, so it's probably
not worth refactoring.

For SWDEV-543208.