Common backends (LLVM, SPIR-V) only supports 1D vectors, LLVM conversion
handles ND vectors (N >= 2) as `array<array<... vector>>` and SPIR-V
conversion doesn't handle them at all at the moment. Sometimes it's
preferable to treat multidim vectors as linearized 1D. Add pass to do
this. Only constants and simple elementwise ops are supported for now.
@krzysz00 I've extracted yours result type conversion code from
LegalizeToF32 and moved it to common place.
Also, add ConversionPattern class operating on traits.
