LLVM IR lowering: support 1D vector operations

Introduce initial support for 1D vector operations.  LLVM does not support
higher-dimensional vectors so the caller must make sure they don't appear in
the input MLIR.  Handle the presence of higher-dimensional vectors by failing
gracefully.

Introduce the type conversion for 1D vector types and hook it up with the rest
of the type convresion system.  Support "splat" constants for vector types.  As
a side effect, this refactors constant operation emission by separating out
scalar integer constants into a separate case and by extracting out the helper
function for scalar float construction.  Existing binary operations apply to
vectors transparently.

PiperOrigin-RevId: 225172349