Fix some 16- and 8-bit behavior in x64 backend related to rotates.

Uncovered by @bjorn3 (thanks!): 8- and 16-bit rotates were not working
properly in recent versions of Cranelift with part of the lowering
migrated to ISLE.

This PR fixes a few issues:

- 8- and 16-bit rotate-left needs to mask a constant amount, if any,
  because we use a 32-bit rotate instruction and so don't get the
  appropriate shift-amount masking for free from x86 semantics.

- `operand_size_from_type` was incorrect: it only handled 32- and 64-bit
  types and silently returned `OperandSize::Size32` for everything else.
  Now uses the `OperandSize::from_ty(ty)` helper as the pre-ISLE code
  did.

Our test coverage for narrow value types is not great; this PR adds some
runtests for rotl/rotr but more would always be better!

cranelift/codegen/src/isa/x64/lower/isle/generated_code.manifest

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