[mlir] Narrow bitwidth emulation for vector.load

This patch is a following for the previous patch https://reviews.llvm.org/D151519.
With this patch, vector.load op with narrow bitwidth (e.g., i4) can be converted to
supported wider bitwidth (e.g., i8).

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D154178