[LLVMCPU] Add vscale_range target field, require it for scalable vec (#24848)

In-lieu of the feedback on https://github.com/iree-org/iree/pull/24698,
this PR adds a user-specifiable `vscale_range` that is recorded on the
executable target and that also stamps the dispatches. This will be used
for lowering later on, specifically while setting tile sizes and
aligning them to scalable vector sizes.

For targets that support scalable vectorization, specifying this flag is
a requirement if scalable vectorization is also enabled. If the target
has `+zvl*b` features and the user specifies a lower bound, the maximum
of the two lower bounds will be effective.

Assisted-by: Claude Code

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Signed-off-by: Ege Beysel <beyselege@gmail.com>